Players will continue playing for as long as they can afford to and are still enjoying the game (or are still addicted to it).
What you might be noticing is that players who are more addicted to the game tend to play it more often, while those who aren’t addicted play less often. (The neat side-effect of this is that if your players aren’t addicted but are happy, they’ll keep paying subscription fees but won’t be using up space in your regions or be demanding new content nearly as often as the addicted players do! If your income is mostly from subscription fees, then having non-addicted players is actually kind of ideal!)
In general, if you look at a player’s “addiction” statistic (when you select them it’ll be visible over on their “stats” tab), players will tend toward playing a number of days per month equal to their ‘addiction’ level (minimum 1). But those play schedules change slowly and randomly over time. But if you were to put the game on fast speed and nobody’s addiction level changed, then after a couple months you’d probably find that everyone with addiction level 3 was playing three days per month, for example.
(note that at the moment, the game simulation says there are just five days per month, so someone on maximum addiction (addiction level 5) would eventually be playing every day, if they maintained that addiction level long enough)
We definitely need to have more visibility into that, somehow! It’s totally not visible enough in-game right now to explain what’s going on.