Draw as much as you want to. Art isn't a race to see who can become the best the fastest. Unless you have a specific reason that you need to improve your skills drastically in a set period of time, then just worry about enjoying yourself.
If for some reason you do need to get good fast, then honestly, time invested is the least of your worries. Yes, you do need to draw to get good at drawing, but as long as you are drawing, your research skills and your ability to push yourself counts more than the amount of time you invest.
When you practice it's because you have a goal in mind, but the thing I've found in Art is no matter how much skill you get you can never settle down because there's always something new to try.
I'm always learning and growing, but that's a side product of the time I spend designing and drawing. So if you call it practicing then I'm always practicing.
Time I spend drawing daily, anything from nothing to 10-15 hours depending on what I'm working on at that moment and whether I'm getting paid or not. As 'working' goes I consider any time I spend looking at art, part of my work, because I'm analysing and deciding what that piece I like does that I don't and how I can get that so even then you're 'practicing'.
If you want to get 'good' first enjoy. Draw what you like. If you love going to the Smithsonian and drawing the things that's fantastic. I personally prefer going to a public place and drawing the people, (I doodle on the bus, and in cafes and just in parks) because I like drawing people. But I know friends who go to places with nice architecture to draw the buildings. So do what you love and enjoy it as well as take it seriously and you'll get there
I say per day I may do 20 minutes sometimes more, sometimes less; I can't do that much because I have a job and many other things to do, so I can't spend hours drawing per day, just sometimes on the weekends I may draw for, I don't know, maybe 5 hours.