Look for a natural stopping point
The best short-session games end themselves. An energy bar, a daily task list or a production cycle gives you a clear moment to close the tab without feeling you have abandoned something.
Sunrise Village and Hero Wars both do this well. A persistent strategy world such as KingsAge does not — it will happily absorb an entire evening.
Beware of scheduled events
Many otherwise-flexible games run timed events that reward being online at particular hours. If your time is unpredictable, prefer titles where events are optional rather than central to progression.