The answer to the question of how much piano practice should a person do can vary greatly. If you have a piano teacher then I would recommend going with what they advise simply because they know you (or will get to know you) and I don’t!
If you are a beginner then I would encourage you to practice for 5 to 10 minutes everyday. When I say everyday, I do mean every single day. Obviously there are things that can happen that you don’t get to practice every day, but the ideal is certainly daily. The reason for practicing daily is to help get you into a routine of practicing, where piano practice is not something that you think about doing daily, its something that you actually do every day. When I teach older kids that have a bunch of activities that tell me that they can’t practice daily because they don’t have time, I ask them whether they clean their teeth daily and reply that of course they do. I explain to them that they automatically clean their teeth each day because they are in a routine and that they never stop and contemplate whether they are too busy or not.
If you genuinely can’t practice everyday, then still try and develop a weekly routine where you practice several times a week – it saves having to think about it or plan for it.
The amount of time I would recommend for a beginner is between 5 and 10 minutes. If you want to do more than this, then that’s absolutely fantastic, but don’t worry if you can’t do more than this at this stage. I also think that it would be more beneficial to do short piano practices of no more than 10 minutes at a time. This will help to maximize your focus and concentration. For example, you would probably get more achieved as a beginner if you did 3 practices of 10 minutes each rather than one 30 minute practice. However, if your preference is to get all of your practice done in the one session each day, then stick with that.
Once you progress from the beginner phase, I would encourage you to gradually increase the minimum amount of time that you practice each day. When I was a piano student, I started at 5 minutes a day practicing and then with each year (or thereabouts) my teacher would increase my minimum practising time by about 5 minutes (which really felt like nothing). So by the time I was in my last year of high school, I was practising an hour a day, but it didn’t feel like it.
Obviously I am talking about the minimum amount of practicing here, but if you want to do more, then definitely do more because the more you practice the faster you will get to your ultimate goal of playing all that you want to play on the piano.