I think the biggest drop I've taken on a hardtail was about 2-3ft. It was a drop to flat, my tires were underinflated, and I got a flat spot in my rim from that. That's about the max height I'm comfortable with on my FS these days, too. All that previous talk is regarding drops to flat.
When it comes to drops to tranny, my comfort level extends a bit because the landing is much smoother. It doesn't jar the body and it's not as hard on the bike. There's really not anything bigger than about 2' on my local trails, though. We have one elevated structure that has a drop to tranny on the exit. The total drop right off the lip is maybe a foot or a foot and a half. If you have much horizontal momentum on the exit, that drop can be much more (the structure is probably 5.5' high). We have some jumps that you can get MAYBE 4' of air on, but the landings are almost all flat ones, so I scrub a little speed before hitting them.
I've just never been all that comfortable in the air, so I keep myself from going overboard.