So, do I need to take a break from guitar often? I find myself sometimes playing a month in a row, every day, without a day off. How often should I rest, and how many days? Is one day enough to rest muscles or should I sometimes take even two? And can not taking breaks slow down my progress?
An hour or two really shouldn't create problems your body can't repair by the next day. These aren't terrifically high impact activities. Make sure you're stretching out your fingers and wrists before and after, and being mindful of whether you're experiencing pain or discomfort. Those are signs you need to stretch, ice, and rest.

However! If you're playing guitar a fair amount, and then going and typing on a keyboard all day long at work or school, you're really asking for carpal tunnel. The biggest thing is to make sure your motions are relaxed and your technique is good, for guitar and for other activities. Make sure you're sitting up straight and have your wrists flat while typing, etc.