M6. But I wouldn't simply replace them. Then you'll have the opposite problem in that you'll have a heckuva time getting the seat OFF.
There are a couple of threads for ideas on homebrew quick-release seat bolts or pins you might want to look at.
Have a look at the inserts in your seat to make sure there isn't half of an old bolt stuck in there. A lot of times if the P.O. left the seat bolts loose they became sawn in half and the stub is still stuck inside the seat.