You will typically receive a minimum of 4 seasons at a DIA school when you move up from DIAA. After 4 seasons, the school expects you to have the program in the position to compete for the conference championship and winning bowl games.
In more general terms, the initial 4 season "contract" is extremely realistic, especially for schools within BCS conferences and coaches can save their jobs and essentially get 1 season "extensions" simply by winning a bowl game. A coach can get a much larger extension (jump back to very secure job status) by winning a conference championship or a BCS bowl game.