I think with even 15 minutes a day you can move the ball forward. Especially if you do longer practice sessions on your days off, to sort of anchor your learning. That's what I tend to do. Once or twice a week I sit down and really dig deep into learning something new. I may not be able to play it up to speed but I know it. Then the rest of the week I chip away at it a few minutes here and there. Don't give up!
- better to play regularly for a short time (like 20 minutes a day, or even better: twice 20 minutes a day) than occasionally for hours on end.
- set yourself a goal and work towards it.
- beware: don't practice your own mistakes. Before you learn something, make sure you have it right.
- record yourself and try to find out what's right and what's wrong
- try to practice slowly, but regularly. Use a rhythm device.
- make sure you know the melody line (i.e. be able to sing it) before you start playing it.