It's 30 minutes delay, but if the posted delay is 25 the gate agent and I both know that will exceed 30. I have gotten free standby on virtually every significant delay, except for when I preferred to keep my "A" boarding pass.

Note that when you keep your seat on a delayed flight it is often quite empty, allowing you 3 seats to sleep on the whole way. That can result in a net savings of usable time.