Is there any regulation that specifies how often soldiers MUST (not recommendations) have access to showers in a field training environment?

Bathing requirements in Field Manual (FM) 21-10, Field Hygiene and Sanitation, state that
optimally, Soldiers should have access to a shower or bath every day, or at least once every week for good personal hygiene. This prevents skin infections and infestation by insects. Given mission constraints, if showers or baths are not available, washing daily with a wash cloth is advised.

SSG (Join to see), as a Preventive Medicine NCO, there are no set in stone regulations dictating the showers, hence the "recommendations." If you are wanting more info, you can find me on Enterprise Global and I will be happy to assist in any way I can.

No, or not that I have ever seen or been provided access to.

How about a refreshing splash in a canal or rice paddy? We could do that on a daily basis.

Unfortunately, no there isn't. In this regard, commanders are given a wide latitude.
Though I'll be honest, I find that since opening combat jobs to women, I feel like there will need to be some regulatory guidance mandating the frequency and availability of showers.

The Last Time I saw any thing was in the 80's in ANOC, and it was 3 days for women and 7 for Men

I always thought a field shower was standing under a 5 gallon jug of water to rinse off, after you were done washing up.

There are quite a few areas that cover showering, I have included below where to find the information and a small excerpt as well. Look into each of the these references to understand it completely. Make sure to pull the information from the Army's Publication site to get the latest information. You can use Google to assist with cross referencing old publications to new ones.


Army Regulation 700–135 Soldier Support in the Field (5 February 2009)
2–6. Showers
Showering with warm water and soap at least every 7 days reduces and/or prevents body lice infestation and positively influences morale. A minimum of 7 minutes will be provided for showering per person. Conservation of water will be observed. Service may be provided more often when extremes of climate, dust, insects, or other conditions cause a need for more frequent showers. The battlefield 12–head shower will be deployed with the SLCR team. The showers will be capable of accommodating both males and females separately. Units receiving shower service will provide the necessary security required for their personnel and valuables while showering. The containerized shower may be deployed as a stand-alone system or as part of the force provider for shower operations.


ATTP 4-41 (FM 10-23) Army Field Feeding and Class I Operations (OCTOBER 2010)
PERSONAL HYGIENE
6-53. The SFOS will ensure established sanitation standards and personal hygiene practices are understood and followed by food service specialists. Food handlers can transmit germs by contaminated hands, soiled clothing, unrestrained hair, and respiratory (particulates from coughing and sneezing). Scratching your head or face, using the latrine, eating, and smoking are just some examples of how hands can be contaminated with harmful germs that are then transmitted to food or food contact surfaces. Commanders and senior unit NCO leaders should ensure the senior food operations sergeant (SFOS) has access to shower and laundry facilities for all unit food service specialists. The field standard is one shower per week, and in hot arid climates, two showers per week. Individuals responsible for handling or preparing food must practice the procedures shown in figure 6-11 to assure proper sanitation and personal hygiene.


FM 4-40 (FM 10-1) (October 2013)
SECTION IV – SHOWER AND LAUNDRY
3-35. Shower and laundry services are performed by field services company shower, laundry, and clothing repair sections. A minimum of seven minutes will be provided for showering per person. Showers may be provided more often when extremes of climate, dust, insects, or other conditions cause a need for more frequent showers.


ATP 4-42 GENERAL SUPPLY AND FIELD SERVICES OPERATIONS (July 2014)
FIELD SERVICES OPERATIONS OVERVIEW
1-4. Field service operations to be discussed in this manual are laundry and shower operations. These services reside within the modular field services companies. They are provided via shower and laundry team capabilities which are projected as far forward in the operational environment as possible. The field service company, located as far forward as the supported brigade combat team, provides soldiers a minimum of two weekly showers and up to 17 pounds of laundered clothing each week (comprising three uniform sets, undergarments, socks, and two towels and washcloths).

FIELD SERVICES CONCEPT OF SUPPORT
5-1. The shower and laundry capabilities are resident within the Quartermaster field services company, which contains six organic shower and laundry teams. These teams are provided from the SBs, normally through the CSSBs, with projection as far forward as possible, and to BCT level if required. The mission is to provide Soldiers a minimum of two showers and 15 pounds of “wash and return” laundered clothing per week. Refer to appendix B of this techniques manual for estimated clothing item weight. Each shower and laundry team can provide support for 500 personnel per day and consists of one shower unit, and one Laundry Advanced System. The laundry unit operates 20 hours per day and the shower unit operates 20 hours per day. Two hours per shift are allocated for equipment maintenance. Personnel augmentation may be required to maintain this operational pace or multiple shower sites. This augmentation is for basic manual labor and assistance with customer flow through the sites. Augmentation does not require additional 92S qualified soldiers. See figure 5-1 on page 5-2 for a sample field layout of a consolidated shower and laundry team operation.


ATP 4-45 (FM 4-20.07) Force Provider Operations November 2014
CAPABILITIES
1-10. Support capabilities of a single 150-person module are:
• Climate-controlled billeting for 150 tenant personnel including Force Provider operators.
• Sanitary climate-controlled showers sufficient for one 10-minute shower per person/per day.
• Sanitary, climate-controlled latrines.
• Food service, to include up to three cook-prepared meals daily.
• Laundry services
LAUNDRY AND SHOWER SECTION
2-33. The laundry and shower section plans and coordinates internal logistics requirements to include the ability to provide shower support on the basis of one shower per Soldier per day. The laundry system has the capability to provide for each Soldier to wash up to 17 pounds of laundry every three days. The Force
Provider modules also include Shower Water Reuse Systems, which gives unit the capability to recycle 75% of grey water from the showers for reuse.
2-35. Each Soldier is authorized one shower per day. Designated laundry and shower section personnel will attend to each shower system to ensure that the facilities are safe, sanitary, and in good working order.
Section personnel will clean and sanitize each shower facility daily and as required otherwise, preferably without interfering with scheduled operations. The Force Provider expeditionary shower system is designed with private environments with separate sections for males and females which enables both genders to use the shower simultaneously.


TB MED 577 SANITARY CONTROL AND SURVEILLANCE OF FIELD WATER SUPPLIES 1 MAY 2010
Contains very detailed information of the quality of water.

Soap and a canteen constitutes as a shower