10 Minutes A Day
When it comes to learning anything practice is required in order to improve. However, I have seen time and time again people saying that they want to learn an instrument really badly, but they just do not have time to practice. At any age, whether it be a child or adult if someone can dedicate 10 - 15 minutes a day to do uninterrupted practice the progress will come, and it may come quicker than you think. It is much more beneficial to practice a little every day than large amounts less frequently.
Sometimes people think they will need to spend hours upon hours every week to ever be able to get to sound really good, when in reality it does not take this amount of time to become a good player. In my opinion, everyone has 10 - 15 minutes a day to practice. If they truly do want to improve on an instrument like many people say they do, then they will find a way to fit it into their schedule.
With all of this said once a person does hit a certain level at their instrument, then their practice time will have to increase in order for them to become better. The best of the best definitely do practice more than 10 - 15 minutes a day, but for someone who just wants to do an instrument as a hobby and as a relaxing downtime they do not need to spend crazy amounts of time practicing every day.