I used to run with just a start time and no definitive end time... but once I ended up in a situation of trying to "sneak in" another session each weak and it was hard-limited to ending at a particular time because people involved had obligations like class and work and sleep before those, I noticed that session was consistently the most productive of all of them throughout the week - people showed up ready to focus on playing, got back to playing in less time when distractions arose, and were clearly trying to get the most out of their time because it was limited.
So I changed to scheduling 4 hour sessions and haven't regretted it yet (it's been working for years now). I do sometimes run less than that, but usually it's because the session didn't start on time and me stopping at the normally scheduled end time, or because some other activity has popped up at an unfortunate time.