I had a thread about this a ways back, but I think one of the strongest contributing factors to having a long term group (aside from general camaraderie and everyone being into it/having similar expectations) is having a clear method for scheduling sessions or handling changes when you have a set schedule.
My group aims for every 3 to 5 weeks and the session ends when have scheduled the next meet up and everyone knows to arrive with a group of possible dates and flexibility for re-scheduling (we usually play on Saturday afternoons, but out next session is a Friday evening) and then short of an emergency, we prioritize that date. We also sometimes bring someone in virtually when needed (as when people had covid exposures).
I think having a DM who has tools for being prepped and not burning out also helps, but I only assume I have those tools because I can't remember even one time when I scheduled to run a game and did not feel up it. Even once when I found out on gameday that a close relative was in the hospital and was very worried but was hundreds of miles away and not in a position to go see them until the next day, I still ran my game. Though in that kind of situation being with friends and having something to do is preferable for me than having nothing to do but sit and worry.
