Ali Ghorashi wrote:
I think the AIM (see 5-1-11) says that flight plans will stay in the ATC
computer 1 hour after departure.

After you depart the flight plan stays active until you land or cancel it.


Also, AIM paragraph 5-1-7 says that: "
Pilots should file IFR flight plans at least 30 minutes prior to estimated
time of departure to preclude possible delay in receiving a departure
clearance from ATC.".

I wouldn't worry about this. If you call AFSS on the radio and tell
them you want to depart now the strip comes up immediately in front of
the controller.