While the server will throttle me if I set it too short (and my code's written to pick the longer of the returned interval and the local setting anyway, of course), I figure it's probably better to be courteous in the first place. I'm figureing something between five minutes/quarter hour as a first cut, but what're other people using?
(BTW, hi, everyone! I'm writing a .NET library to talk to LiveJournal, as I've promised some friends a quick way to port their journals over here from another site. I'm probably going to look at a .NET-based client on top of it as well, later.)