I like two hours when I can afford it. First, you can spend five or so minutes just talking instead of feeling like you've wasted precious minutes. You could spend five more minutes kissing or massaging. You can just take every step and go more slowly with it. If you plan on anal, let's say, I'd make sure you've had at least 45 minutes or an hour of foreplay. And of course, two hours can leave time for some cuddling, etc.
I did two hours recently and the time flew by.