Maybe 15 hours or so. 
Actually I’m not a huge fan of sleeping for that long in the mornings, because for some reason this always gives me a mild headache.
It always goes away almost instantly if I get up, get dressed and have a cup of coffee, though.
13 hours last night, the longest in a good long while >.>
On the downside, it is 3:30 am and I’m not sleepy at all v.v
Umm, that’s a coma! 
This weekend I slept from 4:30AM to 2:00PM, which is the longest I can remember. Apparently, though I don’t really know how this happened, I managed to play about 12.5h of Skyward Sword that day 0_o (and that’s why I need to get back into fitness XD).
But yeah, about 9.5h I guess? Not nearly as long as some people here. I think I might have slept for 10 hours in the summer, but I woke up in between sleeping (this was just straight through).
Really 8-9 hours is my MAX
Just slept 19 hours again.
20h+ once, went to bed at around 7:30, expecting to wake up 20 minutes after, woke up the next day a few hours before, thought I went back in time to be honest.
For about 6 weeks now I’ve been waking up past 4pm 
, that’s holidays for ya.
14-15 hours for me. I tend to wake up because I do not want to waste a whole day sleeping. Although I like sleeping, even I have put limits on myself. (I also tend to get a headache if I sleep for too long, especially mornings. A trait I share with my mother.)
My new record today, 20 1/2 hours~ Hibernating done for the weekend. XD
12 1/2 hours.
22-23hours…i hadnt slept in two days 
Well, I can get nearly that even mostly without a sleep deficit. : P
38 hours (waking up 5 minutes once in a while to go to toilet, eat something). I dreamt a lot. Can’t do that anymore. But 15-16 hours is what i usually sleep if I have nothing else to do the day after.
I was sick once and slept for 22-26 hours :l