What is this "too much" you're talking about?
On a serious note:
I only get headaches when there's something actually wrong. Me not having eaten much, or maybe I'm starting to get sick.
Usually I play (Read: play games or program games) between 5 and 10 hours a day if I have the time.
Really, I think I can play forever, if there is enough content, and if I can switch things up now and then. If I both get to be creative and get to smash some monsters at some point.
At the moment I am in the process of clearing Grim Dawn again, with some of the new content it got. I started playing with a friend around 18 pm... It's 01 am now. I don't plan to stop yet.
EDIT: We stopped at 4:20 am, because we're tired
Not because we've played too much.
Trying is the first step to success.