But most importantly if you are having a busy week and you don't have a great deal of time, even 15 minutes is better than nothing at all. It just keeps what you are currently learning fresh in your mind.
How long you practice does not matter. It is how productively you practice that counts. Simply put it is economies of scale. In other words if I master the final inner part trill of Beethoven's Hammerklavier 1st movement, it is far better than playing the entire sonata through 10 times in a day and mastering nothing. That is the economy of scale. Don't tell me how long, but how you practiced!
on weekends I start playing like... 7 in the morning, and quit around 10 or 11.. with some breaks ofc. On school days I'd say... 7 hours?