I have one group that plays monthly, each session 6-8 hours. Given how infrequently we play, I have essentially gone to a milestone'ish system. Each adventure they get 1/3-1/2 of the experience needed for the next level. Given how infrequently we play, I wanted the PC's to be able to advance and eventually get to level 20 (which would take about 50 sessions/months. i.e. 4 years in real time).
Ironically, in my bi-weekly game, I also prefer for characters to level about every two months.
I have come to the conclusion, that I don't really care how many hours a PC has played, or how many monsters they have killed. I find it more satisfying for both players and DM if there is gradual growth. It lets the players mess around with new abilities/spells and me, as DM, mess around with different monsters and challenges. If we played more frequently, a weekly game for instance, I would probably adhere more to the RAW for experience/level gain.
If I am not mistaken, Adventurer's League will be adopting the checkpoint system from XGtE. 1-4 level each 4-hour adventure, 5-9 level every two 4-hour adventures, etc.