In our current game, the DMs have been awarding XP per session as long as the party is (and... importantly... the
players are) Getting Stuff Done
. Absent characters do not get XP.
We do have some interesting (convoluted?) math involved for determining XP quickly. Essentially, we have awarded XP based on one session to get to level 2, two sessions to get to level 3, three sessions to get to level 4, and four sessions to get to level 5.
After level 5, the XP awarded per session will be the total required to get to the next level divided by 5. For example, it takes 7500 XP to get from level 5 to 6, so each session is worth 1500 XP. Once any of the PCs hits level 6, each session will be then worth 1800 XP (since 9000 XP are required to get from level 5 to 6). Etc.