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.