I’ve mostly had a reasonably quick reply. Quite a few times they’ve responded before my email updates (5 minutes). I have had one occasion where it took over 24 hours before getting a reply.
Copy/pasta of timestamps from my last ticket:
Message from you (Feb 26, 2012 - 02:47:35 / #4774462)
Reply from DreamHost (Feb 26, 2012 - 05:40:09 / #54821079)
~ 3 hrs. Not too shabby at all for $10/month.