How much you practice depends a lot on your goals and your schedule. I believe frequency of practice is more important than quantity of practice. You will retain more and learn at a faster pace if you consistently practice 30 minutes every day of the week rather than 6 hours one day a week.
It is very important to pick up your bass once a day even if it is only for five minutes. Just a few minutes of practice when you don’t have much time will strengthen your memory of concepts, improve your musical ear, and reinforce the muscle memory you are developing.
I also find many students retain more if they are able to scatter their practice time in smaller chunks throughout the day. This can be difficult depending on your schedule, but try it if you can.
If possible, keep your bass out of its case. Make your bass accessible. That way it is easy to pick up whenever you have some time. If you want to force yourself to pick it up, leave it sitting in your favorite chair or in your bed! Put your bass where it is in your way; don’t keep it out of sight.
I think a decent, average amount to practice would be 45-60 minutes a day. If you are consistent, you can make a fair amount of progress with that much practice. If you are more serious about playing the bass, 2-3 hours a day would be a good amount of practice time. And, if you want to be really good you might practice as much as 6-8 hours a day.
The famous jazz saxophonist Charlie Parker spent several years practicing 12 hours each day! Jimi Hendrix never put his guitar down. He even took it with him when he went to the bathroom!
The more you practice, the more you will learn. Of course, you need to be practicing the right things, but more practice helps.
I do think there is a saturation point where you can be practicing too much. At some point you will notice you don't get much more return for the time you are putting in practicing. Don't forget there is a world out there that can really inspire you and make you play better, too. Everything is not in your practice room. Don't be a total shut-in!
As you increase how much you practice, you also need to be aware of the increased health risks musicians face. Long practice sessions may stimulate hand injuries, and you are at higher risk of losing your hearing. Remember to be gentler on yourself as you increase your practice time.
Your practice routine doesn’t always have to be the same length of time each day. You could have a full practice routine (maybe 2 hours) and an abbreviated one (maybe 30 minutes). Choose which one fits your schedule best each day. Don’t skip practicing for the day because you can’t get in your normal amount of practice time. This is a common trap into which students fall.
Above all, if you are pressed for time, be sure to pick up your bass and practice for a few minutes. It will do you a lot of good. Maybe you should do that right now....