It depends on how you practice, but i would say you should atleast take a 5 minute break every 30-45 min or so, just to clear your brain and let the brain process what you just have done.

If you are one of those people that practice with speed bursts beyond your comfort zone i would say it's even more important so you don't hurt yourself. But yeah, a 5-10 min break every 30-45 min can't hurt. As said it lets you process what you have practiced, be it theoretical or technique. It also allow you to "reboot"/take your focus of practicing for a short while so you can go in and focus more later. Trying to sit as long as possible and practice without doing it with focus is pointless.
Fusion and jazz musician, a fan of most music.

Quote by Guthrie Govan
“If you steal from one person it's theft, and if you steal from lots of people it's research”

Quote by Chick Corea
"Only play what you hear. If you don't hear anything, don't play anything."
Personally, I find it beneficial to take breaks every 45 or so minutes. What I've been doing lately is to take three passages from songs that I'm learning and practice each for 15 minutes from half-tempo up to around 80% of their full speed.
-After that, I'll take 15 minutes to either noodle around or just not play guitar for a bit. Time permitting, I'll take the same passages and practice bursts for 5 minutes a piece (basically, run them once at half tempo, then at 75%, then 100%; whatever I mess up on during the 100% run, I focus on doing properly during the 50% and 75% runs).
-After 15 minutes of that, I'll spend the next 45 minutes going from 60-100% tempo (15 minutes a piece).
I was actually more thinking about the physical concept... You know, giving your body a break ?
When I practice, I've found that I hit my mental limit well before my physical limit as far as fatigue is concerned. As long as you vary what you're doing and you're using good technique, physical fatigue shouldn't set in for a couple of hours, especially if you've been playing a while.
I don't know. Sometimes my left arm just feels tensed up. Slight tingling in my left hand ring and pinky finger, weird feeling in my elbow, wrist tensing up when I try to flex my hand upwards (Finger stretching out). My right hand doesn't have any of these issues, so I'm about 99% sure that it has something to do with my left arm technique - which, weirdly enough seems fine from my perspective =S. So far I've seen no harm done to my playing because of this. However, I'm still planning to see a doctor nonetheless, although I'm affraid he/she won't be able to give me some guitar-related advice. That's where you guys come in : )

Some of the things I'm already planning to do from now on:

-Give my fingers some deep pressure massage to get the blood flowing before practicing.
-Stretching both arms - again, for blood circulation and general warm-up - before and during practicing.
-Warming up more extensively.
-Taking breaks every 45 minutes or so.

Little background:
I've been playing a little over 1.5 years - With at least 1 year of non-warm-up, tensed up, crap technique. I don't have a teacher yet, but I'm planning to get one in the next 2 weeks.
Last edited by My Last Words at Feb 5, 2013,
One thing I do if I find my arms tensing up is wash my hands in warm water before warming up and stretching.