I haven't rushed Follies before, but I've rushed a lot of others. And, generally, how early you get there should depend on how bad you want the tickets.
My rule of thumb is if I'm desperate to get tickets and there really aren't other good options for me to get cheap tickets, I go really early. If the box office opens at 10, I get there sometime between 7 and 8, but absolutely no later than 8. If you want the tickets but you don't want to get up quite that early, go between 8 and 9, but risk potentially not getting tickets. I don't know what sort of policies or cheap tickets Follies has once rush is gone, but since it's not selling incredibly well, you could probably get a cheap ticket on TKTS (and probably with a much better location) if you don't get rush in time. But from what I remember reading on here around when it opened, you will probably be fine as long as you get there by 8:30. But someone who's done it recently could perhaps give you a better idea. Hope that helps!
"Art, in itself, is an attempt to bring order out of chaos."-Stephen Sondheim