What's better: pick up a guitar every day for a few minutes or play more rarely, but having a longer session?
Neither. It's not length of time, but what you do with it when you have it. If you spend your time playing the same three songs over and over again, you'll not likely improve save to be able to play three songs endlessly. That's a simplification, but it serves to illustrate a point.
If you're not spending your time working on things that you have trouble with, or working with tools and methods that help you improve, whether you do it for 10 minutes or 10 hours doesn't really matter.
Lets say you have an hour to dedicate to practicing (as opposed to just playing) your instrument, here is how I would suggest you break it down:
- 5m - warm up, stretching exercises
- 10m - modes, scale runs (try to work in things like string skipping)
- 15m - chords, inversions, arpeggios
- 15m - sight reading (I'll pick a new chart from the Real Book)
- 10m - general wanking
- 5m - musical meditation (listen to something new, play blindfolded...something to connect the soul to the music)
Always with a metronome all of it.
The combination of continually stretching your knowledge and your technique and the use of devices like a metronome to ensure you're performing consistently and measuring your success so you can push your limits on the next practice session are what yield results fast.