This is a common question asked by many bass students. In many ways, it comes back to ones goals. Granted, a jr high student may have vastly different goals, than 40+ individual who wants to play bass in their church group. Yet, the goals really do set the stage…. but they need to be manageable goals as well, and likely subject to evolution/review as time passes. Ie, a youngster may start out wanting to be a top 40 bass player, or maybe all they really want to do is impress the young gal next door, or perhaps school work / sports ends up causing ones goals to be re-aligned. The older student may start out just wanting to play in church, but then realize he really wants to get out in the public more, or enhance his skills by expanding the styles he plays.
Initially, there is a wanting to practice a lot, perhaps a bit too much. It takes time for a thicker layer of skin to develop on ones fingers. It does no one any good to end up with bloody fingers and demoralized during week one. Thus, initially, it may be best to limit practice to 15 minute intervals, perhaps even up to a couple times a day, but no more, at least for a week or two.
If one’s goal is to play music for fun and personal enjoyment, and not so for profit or in a group, a reasonable practice schedule is 30 minutes a day. It will keep one learning, and develop skills, albeit not nearly as quickly as a more rigorous schedule.
If a group is where ones heart is, than an hour a day at least for the first few years is really the key. Less than that and progress is not as fast as one would like, and more, unless one has an abundance of time is likely not to produce significantly more value for the time invested.
From a professional perspective, practice time can end up being pretty substantial, with one caveat, and that is the potential for injury. Once one gets much beyond 4 hours a day, even if broken up into multiple sessions, combined with performance hours the potential for reptitive motion injuries skyrockets. Thats an arena no one ever wants to be in, thus care and planning is needed.