Depends on the squad.
In general, I think a 4 to 5 year term for the GM makes sense. Especially now that there is draft slotting. Between that and FA, you should be able to formulate your plan, acquire the necessary players, and execute with expectations of seeing the program flourish in year 4.
As for head coaches, I give them the same amount of time. Except, of course, if they suck really bad. Like if they have a good squad but the squad obviously under-performs or if too many game-day decisions are bone-headed and stuff.
Rookie quarterbacks get 3 years to get with the program. Veteran quarterbacks, 2 years depending upon the complexity of the offensive scheme.
Waterboys get 2 years in case of a water shortage.