I wouldn't bother much more often than every 30k miles, unless its an AT known for generating a lot of clutch dust. ATF has a pretty easy life compared to engine oil- no combustion byproudcts, no acids, no moisture other than atmospheric moisture, no carbon, no sulfur, no unburned fuel / fuel additives. Just clutch dust and gradual depletion of some components of the additive package, that's why they really can live "for life" (and have "life" be > 200k miles sometimes) on a single fluid fill if the owner chooses that level of neglect.