For me, it's tough. I don't know how I can schedule bass into my life. I know, If I REALLY wanted to play bass I'd play regardless of school, friends, personal life, or exams. Trust me, I hear it alot. And the thing is, I do wanna keep playing. Hopefully, even make a poopy High school band. Who knows? How do others do it?
I haven't had the leisure time that I would like to, for years now. I have a big family, a busy primary job, and I teach guitar/bass lessons on the side. Plus I gig weekly with multiple instruments, so I have to keep my skills up on each of them. What I have found is that efficiency, structure, and focused practice are what is more important than overall time. So here is what I do:
I take something that is technically challenging, but not out of my skill set...and use that as my focus. And I practice the technique for a few minutes, then use that skill in a musical setting.
For example. Let's say that I want to practice my galloping triplets or 3 finger technique. I practice that skill with a metronome moderately slowly and cleanly for 1-2 minutes straight. Then I speed it up a little and repeat the process a few times. Then comes the most important part...I play a few songs that use the skill that I am working on. That solidifies the muscle memory. I can keep that skill up in about 15 minutes a day, a couple of times a week. It's even better if the songs that I choose have a couple of skills, because then in about 20 minutes I can work on more than one thing.
Unfortunately, this only works to maintain what you have...it takes a lot more effort to build a new skill.
But, you can structure a practice routine in 20 to 30 minutes for 5 days a week that will improve new skills. You just have to prepare for it and be very focused, instead of randomly playing things.