Both Allied and Axis Main Bases are 3 man capture and 5min grey 10min capture.
For each player in the zone over the amount defending the zone, the capture time decreases. For example, the Axis have 6 soldiers in the zone to the Allies 1. The Axis have a 5 soldier advantage so 5/5min=1min grey 5/10= 2 minutes capture. The times change as the soldiers die or respawn.