I think the reason to do long sessions is to avoid waste of time travelling to/from too often - which is not relevant with online meetings.
So it should simply be down to the most effective time span in terms of learning. Unlike live sessions, online sessions tend to be mostly receptive for the students (ie limited interaction) - my personal feeling is that you very quickly will start to lose them after an hour. And, as opposed to live sessions where you see that, you will probably not be aware of it happening.
And also for you, it gets very tiring very quickly.
I have still not got used to presenting at webinars with hundreds of people in the 'audience'. Very unnerving to drone on for any length of time without any visual or audio feedback at all, knowing they were there - or were they?
So I think 2 hour session are pushing the limit but may be OK if you add variety, take a break, get them to interact etc. I would also suggest that you do ask them to turn on their cameras (unless you have very large classes) and that you often go back to showing your face and not just the slides - or both if your system allows you to. Zoom allows you to have 'polls' as you go along - you could e.g. use these as multiple choice questions as go you along to keep them entertained?
But this is just my opinion - others are sure to have their own and it may also depend on the demographic of your students... Maybe a google on 'best ways to teach online' could give some food for thought?
We all live under the same sky, but we don't all have the same horizon.