I answered over 8 hours. While I try to fit in a one-shot now and then as a player on find-a-game platforms, the vast majority of my game sessions are the ones I run for my 5e campaign. I run a monthly 8-hour game, scheduled from 10 am to 6 pm my players time (currently 6pm to 2am my time). This was true when I ran the game in person from my home and remains the same now that I run the game online overseas. It has been this way since I started running games again when 5e was first released.
I find short game sessions unfulfilling and would rather play board games for any slot under 4 hours, but 6-8 hours is the sweet spot for me. I prefer board games for "game nights
" and TTRPGs for "game days
Perhaps surprisingly, most of the time is spent actually playing the game. When I ran games in person there was often 20 minutes or so spent on players arriving, getting situated, banter, etc. And we would take a break to go get lunch (on sessions we didn't do potluck), but generally the lunch break was 30 minutes at most to go get take out and we generally played while eating. Online even less time is spent on breaks. There will generally be a couple of 10-15 minute breaks, sometimes more or longer, depending. In an 8-hour session a good 7 to 7.5 hours are spent focused on the game.
In long sessions, I find, the players spend more time planning and roleplaying among themselves. We will occasionally spend some of the session on leveling up and downtime activity, but most of that takes place over e-mail between sessions.
8 hours has proven to be the right amount of time in my campaign, with my group, to complete a specific objective. We rarely have to end a game in the middle of a fight or mission. The game is set up so that people can drop in or miss a session without affecting the group or story negatively. Also, with a longer session, we are better able to end the session at a natural end point. So I have ended sessions early when a satisfactory end point is reached, but it is rare that we don't go for the entire 8 hours and more come to go a bit over 8 hours.
Also for us, it is much easier to schedule an 8-hour session than it is to commit to shorter weekly sessions. We schedule the next session at the end of each session. We've rarely had to skip a month and we occasionally play twice in a month. We try to ensure that the core four players can make every sessions, with four other players on the mail list who join when they can. But if one of the core players can't make a session, we'll often still play, but they'll focus on a minor objective, or I'll run a side quest.
I find that my family and friends who are not into TTRPGs find this crazy. But then again, when I was young I used to play war games that would last all weekend. An 8-hour game is nothing to a wargamer.