I normally only shoot with a 1 gig card for speed so if im shooting large file sizes, which I do, It just has to be less than 100ish...
however, If I'm set to shoot wide angle I'll usually come back with less than 30 as theres not a whole lot of chance to mess with the shot (plus theres not a whole lot of wide angle to be had when your average vis is less than 15 feet).
for that reason I normally shoot macro, where depending on location, time and conditions I'll shoot between 30 and 60ish depending on species. Things like nudies and anemonies I like to play around, change my focus, lighting, angle, framing and I can come back with 5 or 6 shots of the same thing all slightly different. theres always 1 or 2 that are bad due to focus, conditions etc, and then 3 more that I cant decide which I like best (and occationally the one that I ask myself what I was thinking when I took it).
on some of the local shallow rock reefs however, I can end up doing a 2 hour dive and almost filling up my card.
then theres also the friends and family aspect:
sometimes when I shoot (particularily someplace new), Im not always thinking in the photographers mind. A lot of times I can come back with 60 shots and 2 of them be intended to look good. the rest just pictures to show off to non divers...like when you go on vacation and play tourist...
why? well you can dive for photography and dive for fun, but when you mix them, sometimes you miss a lot. If your shooting wide angle your less likley to stop and look at the smaller stuff and when your shooting macro you dont always step back and look at the whole picture...we all need days off...
So in the end, I suppose it all depends on my mind set...30-60 is about average to me...