Especially on VBases, Hangar downtime seems to be shorter sometimes, because people witness that Vehicles are disabled less than 15mins, maybe 13 or even a bit less.
But due to the new base layout, hangars are rarely dropped at the same time now. As long as one VH is up, vehicles are enabled. For example, if it takes an attacker 2 minutes to drop all 3 VH's (maybe two passes in a Lanc), the effective downtime will be 13 mins only.