I've never slept for a huge amount of time - probably the most was about 16 hours after getting home from a long and sleepless ccf trip. Usually though, if left alone I can sleep for about 11 hours, then I wake up naturally.
16hrs
Pretty much the same, had a 2 day lash up session in amsterdam for a mates stag and didnt sleep while i was there, came back and passed out for 15-16 hours
On a related note, why does too much sleep seem to make you feel like crap? I have a massive headache when ive over slept