depends on what you are airlifting, (or dropping,) and how many military transport planes you have in unit. recently i had 9500 military transport plains in 190 air transport units, airlifting 8 full bomber fighter wings, it took 8 hours for a double hop, (airlifted twice to get into place,) but usually a handfull of long range divisions will take about 30-45 minutes with 20 atu (1000 mta)
as far as airdropping, if you are dropping like 5ish SF, all you really need in one ATU, but 500 MTA in 10 ATU units would be optimal. and it'll take about an hour. although i've never actually timed it, an hour sounds about right.