My max was ~18 hours, and it wasn't exactly "straight" as I kept waking up for a minute or two, rolling over and going back to sleep. This was of course while I was ill.
When I'm perfectly healthy, my max is 12 hours. Any more than that and I get a horrible sleep headache.
--
Bunny... :3
http://img.imgcake.com/baronvontoast/0413121622bjpguh.jpg