To be honest, I start getting distracted as early as the 1.5 or the 2 hour mark. It is hard to stay focused, but if I feel myself getting repeatedly distracted, then I find that it's best to take a break and not feel guilty about it. I'll take a short walk or check in on a loved one or grab a snack, and that break will help me focus better for another 1.5-2 houes than if I had just powered through. If you don't give yourself a break when you need it, not only will you be less productive than you could have been in the same amount of time, but you might even start to resent yourself and the work you're doing.