First part quoted from post on stackexchange.
In December, 2016, Mike Shea compiled the results of a survey of 6,600 Dungeons & Dragons Dungeon Masters. The survey inquired about a variety of subjects, including frequency of play and session duration.
Frequency of Play
- More than twice a week = 1.9%
- Twice a week = 6.2%
- Weekly = 43.2%
- Twice a month = 25.9%
- Monthly = 13.3%
- Less than monthly = 9.6%
- About an hour = 0.5%
- About two hours = 5.0%
- About three hours = 28.4%
- About four hours = 43.9%
- About six hours = 16.7%
- About eight hours = 3.7%
- Longer than eight hours = 1.7%
Note that these survey results are focused on Dungeons & Dragons, and the usual caveats about selection bias apply. The survey is a treasure-trove of information though, and Shea has taken great pains to make all the underlying data accessible.
An important thing to note, is that in the end it is down to the GM, given the players availability, how long sessions should be in their campaign.
GMing is a lot more demanding and tiring than playing a character in the same session. So even if the players might be willing to keep going, a lot of the times the GM will be spent well before the players are. This is due to the nature of GMing, which involves among other things having the responsibility of keeping track of everybody and everything over the whole session, keeping the players involved, creating a lot of content on the fly in reaction to what the players want to do, moving the narrative forward, looking up rules and tables in books and so forth, the list goes on. These “duties” go on more or less constantly during a session, where as players are only concerned with their persona in the game and often get times where they are not actively engaged. So overall it is much less demanding and relaxing to play a character in a campaign than to run it.
Other things that effect this are how your play. If you play face to face, in which case the GMing is at its best as you can read the vibe around the table and from individual players faces which aids in running the game. Those sort of meet up also often include spending some time socializing outside of the game before and after, and possibly a shared meal.
If you’re playing online on the other hand, on places such as roll20.net this interaction changes a lot. The sessions become a lot more “compact” so to speak and the pressure levels rise as a result on the GM, as its more likely to be all down to playing from the get go and until the end of the session, with a possible bio break or two. But spending time relaxing and generally socializing during this time, let alone taking a meal break is less likely. Add on top difficulties such as varying degrees of voice quality and possibly not being able to see your players if there is little to no video, makes things more difficult and stressful.
So play time per session is likely to decrease in relation to those added pressures.
But in the end it is up to each GM and their group to find what works best for them!