How many minutes/hours do you really sit to write code at a particular moment?

I'm not saying the total summation of hours you code a day. When you really sit down to write a code for a particular task at a moment, how many minutes/hours (at worst case) do you normally sit down before you get tired? I know some take break, some say it depends on the task or the individual, I would love to hear them all, and what you do to keep your brain refreshed before getting back to coding. Thanks...