Is there a limit to how long you can do one tour, because I heard someone say they did 15 months in one tour?

Back during the surge in Iraq several units got extended to fifteen months. The goal was to raise troop strength to over 100K. My unit with the Texas National Guard was one of those units along with many Active Component units. The only limit is the requirements of Army leadership.

The saying when I was in was"AF needs come first". So yes, some get screwed based on situations.

Well, this depends on what the situation is.
If it is a declared war then Soldiers could stay for the duration of the conflict plus whatever length of time Uncle Sam determines is appropriate to stabilize the situation.
In current and recent operations, Congress authorized the President to deploy Reserve Component troops for up to two years at a time, although it hasn't happened to my knowledge. AC troops could hypothetically stay longer than they have, but most rotations (in the Army) were between 9 and 12 months. The Combatant Commander makes this call. USMC and USAF deployments have been somewhat shorter.
I myself have done four deployments, and they were 8, 14, 13, and 10 months in length.
My coworker right next to me was in the MN NG during the surge, and they got stuck being activated for 22 months as their luck had the timing just so. 17 months of that was in-country.

It happened. Back at the end of OIF-1(April 2004-ish) a number of 1AD units were turned around from heading home after a full year on the ground and sent back into the fight. HUGE morale killer for a lot of folks who were already spent and had washed, turned over/in their vehicles, gear, and even body armor(there wasn't enough IBA to go around back then). Had our company not partially left for home already, we would have been in the same boat.

During Desert storm my tour in Korea was extended from 12 months to 15 and again in OIF several units had their 12 month tours extended to 15 months. There is a lot that goes into the decision to extend tours, mostly depends on mission load and unit availability.

Will-There are several reasons for extensions; some involuntary, others voluntary. At one point during OEF, different services had different "tour lengths", and all of the above changes with the needs of the Service.

My unit in Afghanistan was on 12 month orders, but was offering individual extension...I believe one guy did two tours back to back, though that was admittedly rare.

Depends. I did a back-to-back 18 months in Vietnam...then two 20 month tours in Iraq. If you're deployed and want to extend, then put in your request as soon as it's acceptable. Guidance to me at the time of my deployments was that I could request an extension at six months and one day (Day #181) - but that was probably local policy. Easiest way is to stay close to Personnel and determine the "extension" guidance for your particular tour.

I assume you are talking about a combat deployment, not an "overseas tour." There is no limit on combat deployments. Typically they have been for a year. That's not mandated in law, regulation or policy. There are restrictions on National Guard and Reserve call-up though.

When I deployed I knew people that did years overseas but most of them volunteered to stay with another unit and with their command approval; the most I heard of a single unit doing a deployment was around 15-18 months. So you can actually stay there a while

My unit (172nd Stryker Brigade) was extended and my total deployment was 16 months from start to finish. Mid-tour leave was not removed from anyone so for me personally was a total of 15 months on that specific deployment. My other deployments were 1 year with mid-tour.
The point; "Mission comes first."