In my RL group and 50% of the online groups I play we just level up, when the GM says so. In the other 50% of the online groups we usually get 1/3 of the XP needed for the next level. If a player misses an evening he gets a 50% bonus until he is around the xp of the rest of the group.
I started to prefer to first option, because I never really cared about the number of xp I got.