How long do you need to feel you've had a proper session?
When I was younger I would go for anywhere from 6 to 12 hours, or longer if able, without batting an eye. The gaming would just happen. It was a great time, though after about 8 hours I'd be willing to question just how well I was actually managing the world, the story, and other expectations on a GM.
Now, I think the standard is 3 to 3.5 hours for the groups I'm in with the occasional session going longer as needed to resolve a fight, scene, or adventure if we're almost done it.
The thing is, I find myself wanting more time a lot in those 3 hour sessions. Digressions - and they do happen - are so costly, and it can feel so hard to get meaningful progress done when you only have a few hours. However, longer sessions are often impossible to fit in or get people available for. Life is busy, people are busy, and 3 hours is a lot easier to fit in than 4 hours it seems.
On the other hand, one game I am in regularly runs for more than 5 hours when it runs. Sometimes that is great. Other times, well, I'm trained by all the 3 hour games that it can feel long, almost like forever. And someone help me if I slept for crap the night before and my character isn't front and center for a prolonged period of time.
I think my ideal is still 4-6 hours. It gives enough time to get stuff done. It gives room to digressions. It is long enough to justify a small break for bathroom, snacks, etc. And you have flexibility to go a bit longer without feeling like too much is being given up.
Of course, that means the only time I can do an ideal session is Saturday or Sunday, which is also not ideal. At least for my life. How about you? What's your ideal?