There was one time me and my ex bf did for 2 hours straight,
just a little pain and no other side effects. Idk, that's just my
experience.
2h straight? Lucky you... 
Did read on Women's Weekly though, that someone's hubby did her
for like, 3h or so straight...... and it took her more than 1/2h of
BJ (not sure if it was a continuation or standalone) to finish him
off......