Two hours seems about right. It is substantial enough to get a lot of work done, even with a few distractions. It's not so long that you need to get to make coffee or go to the bathroom. You have time to warm up and then cool down at the end, and switch focus a few times. If you don't get through the whole two hours, you're likely to have worked at least an hour fifteen.
String these together on successive days over a week or a month and and you will be unstoppable.