I have read here about 7-9 fullsizelevels (so 1,85gb) but think wolf has done more, otherhand 20+ 900mb levels. but as said, i think there is simply more to calculate in bigger levels where you or the engine can get in trouble with.
my personal thought (not experience) is that its possible to get 20+ levels with this, IF the levels,scripts, models etc. are perfectly cleaned and arranged in the right way (think of variables or wrong-and-messed-but-working-things in scripts) ... and IF fpsc has a good day.
out of my experience fpsc does another job with each build. one works, the next without any editing does not. so its worth to build more than 1 version in a session to give the engine a second chance.^^
justmy2cents