I think it depends on how frequently you play and how long you expect the campaign to last. I generally run campaigns with the expectation that they're going to run for many, many sessions; so I tend to try to err on the side of too few XP rather than too many. I've been handing out between 6-8 a sessions; and thus far, its been keeping the development pacing about where we all want it.

I always give a base of 15XP each 3-4 hour session. If they did some particularly amazing stuff, I'll tack on an extra 5 or 10. The worst that happens is their characters become TOO COOL.