Hmph. I just went and gave a few people some green reps for a thread I liked and ran into the "You have given out too much rep in the last 24 hours" message. I had no idea there was a limiting system in place.
That's one of the standard options in the vB rep system. You can change it too (users can give more or less within 24 hours).
(I think this should be in Suggestions & Feedback) Anyhow, yeah I think its some low number.. maybe 10?