Flags will despawn 20 days after they are raised unless they are manually raised before that time.

These items are protected by a flag. When/If the flag drops all the way, the flag will despawn. These are the lifespans for items when they are no longer protected by a base flag:
Vanilla Base: 45 days
RaG Hunter Cabin: 14 days
RaG Base Building: 14 days

Lifespan for vehicles including cars, boats helicopters and bicycles: 7 days.
Vehicles parked in virtual garages have a 45 day lifespan.
Vehicles are NOT protected by a base flag.