Field objects (ammo, radar, fuel tanks, auto guns, town buildings, barracks) stay down 30 minutes.
You can increase downtime by killing the corresponding factory before porking field or killing town. A factory/city at 0% increases downtime by 120 minutes.
The only thing I don't know for sure if there' still a working convoy system these days, haven't seen any trucks since I rejoined 2 months ago. If so, killing a truck/barge would add a small amount of downtime to already destroyed field objects.