How much time you spend practicing will depend on how much you enjoy it and how good you want to be. If you really enjoy it the hours will fly by and you won't need a timer to work to......If you don't enjoy it, any amount of time is too much!
As with all learning, it's usually a case of 3 steps forward, 1 step back in the early days, in other words, you will learn stuff at a lesson then, unless you practice that lesson often enough you will forget some of it. The less you practice, the more steps back you will take between lessons such that your overall progress over time will be greatly diminished.
When I started learning the piano some 25 years ago, you couldn't get me off the thing. I would spend at least an hour a day, and often longer, at the piano. As the years go by I tend to spend less and less time playing and my progress has halted as a result.
The only real answer is to practice for as long as you enjoy it. As soon as you stop enjoying, you will turn it into a chore and lose the love for the instrument.