Timing your breaks is kinda like doing reps and sets in weight training. Lifting a certain amount of weight you do a set of 10 reps and then you wait for minute or so, then you do another set and wait for a minute or two and so-on. Too much time between sets is bad and too little can be worse.
Here's the deal - in this guitar thing, you're the only one who can sense when it's time to stop. So, here are some indicators.
Sometimes you get an anxious feeling about the whole practice effort. Your will to practice is strong but an anxious or nervous feeling distracts you from being able to focus on what you're trying to do. And remember, focus is important.
When this happens it can be good to do something that completely takes your mind away from practicing altogether. Go for a challenging run, go play a game of basketball or a round of golf. Anything that will help you to get rid of this nervous energy.
Most of the time you'll come back having a more relaxed and hopeful attitude.
Frustration is different from anxiousness or nervousness. Frustration can be born out of a feeling that you just can't do whatever it is you've been struggling to learn. It's just never gonna happen.
This is a dangerous point in your development as a player because this is when most people quit. It's an emotional decision that goes something like this - "I've tried and tried and I just can't do it! I'll never be able to get this down so why do I keep punishing myself!? I'm done. I've had enough. I'm outta here!"
Rather than allow this feeling to bring you to that point, stop. Put down your guitar and go do something else for a while. You have to sooth the savage beast inside you!
Come back even as little as fifteen minutes later and you may find that you can pick your guitar up and immediately do whatever it was you were struggling with for an hour or two in your prior practice sessions.
If you lost the ability to focus on what you're trying to learn, you might be experiencing a little mental overload. I'm no expert on brain matters but I know that many things take a little time to absorb. You might be right on the verge of really grasping a new concept and playing it but you just can't get there because your brain has decided it's just had enough! It says, "Ok, I'm shutting down now." - your brain stops absorbing anything.
Again, the advise is the same - take a break and occupy your mind with something completely different.
Many folks think that, to give themselves a mental break, they've gotta go lay on the couch and zone out for a while. That's not really the best way to get a mental break in this scenario. If you do that, you're gonna find your mind drifting back to the problem at hand. You'll get into a conscious frame of mind where you are still trying to solve the mystery of why you can't execute this new thing on your guitar.
Notice I said a conscious frame of mind.
Your unconscious mind is much more capable of working through these things than you might imagine. Subconsciously (referred to these days as the unconscious mind) your mind will continue to work on these things while your conscious mind is on something altogether unrelated. It's a pretty wonderful thing - this mind with which we have been gifted.
If you want to find some good reading on this phenomenon do a search for "does the mind unconsciously continue to work on a problem?" and you'll find several articles that discuss the reality of the power of the unconscious mind.
Again, if you occupy your conscious mind to work in a completely different, challenging arena like playing a game of golf or whatever that is for you, you are forced to focus on whatever that is. That forces the problem you are trying to solve to flow into the unconscience giving the unconscience mind a chance to work on it for you!
For me that's a round of golf or banging balls on the driving range. The whole time I'm there I'm analyzing what I'm doing wrong and trying to fix my swing.
I've come back after a round of golf or other mind-occupying activity, picked up my guitar and somehow was immediately able to play the thing I just couldn't get before. It's pretty cool when that happens. You should try it yourself!