Is there a guideline for how many hours a soldier can be on extra duty per day (e.g. # of hours on a duty day, non-duty day, etc.)?

Never saw a G.I. punch a time clock.

"performed at anytime" and "any length of time" so a Leader must select wisely. ArmyFit educates us about getting appropriate sleep. Too much work over too long a time can lead to burnout.

AR 27-10
3-19 Rules and Limitations
b.(5) Extra duties. Extra duties may be required to be performed at anytime and, within the duration of the punishment, for any length of time.

25th Infantry Division just put out a new memo stating when the hours for extra duty are to take place, as for the amount of hours a soldier must perform extra duty per day I believe is under AR 27-10

As far as I know, the number of hours is dictated by the commander who imposed the punishment, often delegated down to the 1SG. From what I remember, it was usually 4 hours, beginning 1 hour after the unit was released for the day. Eg; unit released at 1700, 1 hour for chow, report for ED at 1800, end at 2200. CQ or SD would inspect the work before releasing. If ED tasks were not satisfactory or incomplete, they were not released until it was. Also, back in the day, some 1SG's and CSM's dictated that ED was performed in full battle rattle, but that went away sometime in the '90's. Weekends were 12 hrs (8 hour duty day + 4 hours ED). That is just an example, I don't think it's in the reg, just covered by unit policy/cdr's discretion.

20 hours....Only required to give you 4 hours rest & it doesn't have to be consecutively...

No...
When I received A Field Grade AR15 when I was stationed in Germany, I received 45/45 for getting into a fight with 3 germans nationals in Garmisch, Germany, one didnt fair to well. My day started at 0545 I got off duty from the motor pool at 1700, was allowed 30 mins to eat chow and had to report to Staff duty by 1730, luckily I lived on a small Kaserne/post. My day ended whenever Battalion was completely cleaned from top to bottom...3 story building or Midnite, which ever came first, luckily I worked from the time I got there till i was done, never was dogged by SDNCO, it was a long 45days, but, I got it done. On the weekends pretty much raked leaves, from 0700 to 2200hrs just depended on Staff duty.

In two units I was in, it was either 23:00 or 0:00 (depending on my unit at the time) that the extra duty got off. Then they would be required to be up the usual time in the morning. As far as I know there is no regulation about it.

I have usually seen kids stay after work late to clean the squadron buildings for a few hours followed dorm detail for the rest of the night. On weekends they were given base cleanup details. I think the idea was to loosely take enough of their free time away when they would rather be doing something else like playing X-box in their room.

There is no restriction on the amount of time. However punishment cannot be cruel and unusual, nor can it be demeaning. Like a recent post with a soldier mopping up a parking lot (motor pool) in the rain. You must also use caution when it pertains to sleep depravation which also lends to cuel and unusual. A best practice is to try to fit the crime or have the service member in question perform those menial tasks during extra duty that would otherwise have to be done through a detail during normal hours.