Sorry to revive an inactive thread but after PMing theymos, here's his response (with permission to publish it, obviously):
The strong guideline is 1 bump per 24 hours per thread. Additionally, bumps should not be annoying, which needs to be determined subjectively. If a user bumps 10 threads every day, that's annoying. If a user bumps 3 threads at the same time for some good reason, then this may not be annoying.
When it comes to evaluating what is "annoying", it's up to the moderator who's looking into a case to decide.