Great question. I actually make a list of things to practice every week. Every week I have a set of objectives given to me by my private instructor and different assignments for school. I believe this method is the most efficient way to learn, memorize and practice material. For every objective on my list I have a set interval of time. Here's exactly what I do.
I wake up every single day and hit the practice rooms at 7:30 am. On fridays it's 8, and on Sundays it's 9. I never miss a day. When you start missing your sessions, your schedule will slowly but surely fall apart. Once I am in the practice room, I set up my laptop on my right side, a music stand in front of me, and my jacket on the door handle. I may be crazy, but this kind of consistency is what I have to do in order to practice correctly.
I set my first timer for 15 minutes, and I usually go through something that requires absent minded repetition in order to be able to play what ever it is at tempo.
I set another timer for a transcription I am working on, Again 15 minutes.
I set a timer for fifteen minutes and work on some cool lick or passage I am working on. Usually at a slower tempo, if I get antsy I'll up the tempo by 4 or 8 bpm. It's incredibly rewarding to be able to play things fast.
Again, 15 minute timer for the mindless repetition.
I take a break for ten minutes.
I go back repeat the hour long cycle.
break for ten minutes.
Then I start figuring out the next part of the transcription I have to learn for how ever long it takes. After I have it figured out, I usually practice that passage for thirty minutes at a very slow tempo.
I'll piece the two parts of the transcription together till I can play them without fumbling or hesitation, than I'll play that for 15 or more minutes at a slow tempo.
I take a short break if I feel like it.
Then I'll read violin studies for twenty minutes. I hate this part.
I repeat the cycle of the first hour.
If I have time left, I'll play the full transcription that I have learned so far for the remaining time.
Yes, it's a lot. The world of guitar is incredibly competitive, and this is truly what it takes if you want to teach at the collegiate level, or get gigs that you can actually survive off of.
If you want to get better than your friends I suggest practicing these few things:
Scales, including the modes. Start in first position and work your way up. You usually do not start on Ionian to execute this. Reading. It sucks, but it's a must. Get software that allows you to quickly create backing tracks such as iReal. Get the amazing slow downer. Steal from the best. Always play with a metronome. Always. Don't practice mistakes. Be consistent and humble, because you're going to surprise yourself by how good you're getting. It doesn't matter how good you are- if you're a dick about your chops no one wants to play with you.
Good luck !