I once read or heard somone say that on the astral level time does not exist, allowing one to interact with beings that have not yet been born or have been dead for quite some time. This brings up the question, is it possible to spend an eternity on astral level in say, one session? Maybe not an eternity but longer than it would actualy be in the physical world.
Wherever time doesn't exist, time can't pass, so time can't be "longer" (description of time) than anything anywhere. Your current place and time in the physical world would be both minuscule in the whole of things and stretched into infinity.
To add to what Stookie said, most people tend to think that time is an 'object' that is 'doing' something, while time is just a measurement of something that is happening- most likely the expansion of space and the changes that happen when space expands. (IMO)
So time may seem different in the astral- I have had long periods only take a few minutes- but different is not nonexistent.
If I were going to take the metaphysical view (which I often do) I'd say that we are already spending an eternity in the nonphysical, but experiencing it as some form of physicality- from the density of the material all the way to the nondensity of the mental.
But if there's perspective of any kind, chances are there's time, in the astral, the buddhic, and even the mental. What happens 'aftewards' is probably an oxymoron.