Looking at http://marvel.com/universe/Attilan the Earth-616 Inhumans have been inhabiting the moon for a relatively short period of time: seven thousand years ago they began to live in isolation on an island in Atlantic ocean. Then in mid-20th century they moved to Himalayas. Some time after that(possibly in the 1980s) they moved again, this time to the Moon.
However, in the new TV series, which are probably in MCU I get the feeling that they have been on the moon for generations at least. Is there any more precise information on how much time has passed since their exodus? A century? Millennia?