Greetings.15-20 min. practice sessions? How is that going to be effective? All I can think of is spending that time on a couple of bars otherwise you can't get anything done.
If you restrict yourself to timed practice, such as the 15-20 minutes thing, then you will not only not get anything meaningfull done, but you will be distracted by the clock. I like to just practice for as long as it takes, not worrying about the time.