2 years is my preferred length for these kind of games. Only a year for anything at a physical table. When it comes to MUโs, I notice that I take about a 2 month break after a year or so, because I run out of things to do or just need to step back.
A lot of it comes down to how the gameโs progress works and how much happens in a given stretch of time. Like @tsar said, its about number of players, turnover, and how staff handles the setting.