On several forums recently (not this one) I've encountered a time limit on
searches. Now that, I don't get. If the first search doesn't turn up what I'm looking for, of course I'm going to do another one right away...
I guess there could be trolls out there spamming searches to overload a site's server (uh, maybe?), but it seems to be at cross purposes. If you prevent someone from searching, you're just forcing them to post something that could be repetitive (and therefore considered spam by many of the people who read the board regularly).
I remember being surprised the day I learned PMs had a length limit. I guess I was particularly verbose that day.