I go by a set schedule - for the one group, we game on the first and third Saturday of the month, and the other (which has shorter sessions), every Thursday. As long as there's at least three people that show, we game.
Back in the 4e days, I used to only run when everyone could make it. We were gaming only once a month back then, and there would be months where we just couldn't agree on a time, which lead to missing whole swathes of months. I eventually gave up on that approach. I've learned that missing sessions leads to more missed sessions.