I was talking with one of my adult students this weekend about how much practice time is appropriate for a beginning flutist, and a point came up that I wanted to share here.
When you're learning anything new, the novelty of learning is what keeps you coming back to practice and get better at whatever skill you're trying to acquire. With this being said, QUIT WHILE YOU'RE AHEAD.
What I mean is this: when you are a novice at anything, you should end each practice session while you still feel joy and excitement in your heart. If you go to the point where you feel frustration and disappointment, you'll burn out and not want to continue learning. There will be plenty of time down the road to work through tough stuff even though it's not the most appealing thing to do, but the time for that is not when you're just starting out.
Lay down a foundation of love and respect for your instrument, because you'll need that to draw upon when the time comes to work on things that are beneficial but not all that fun while you're doing them. There will come a point when you need a reason to continue slogging through that etude or to keep working to get that technical passage just right, and the thought of how much you love what you're doing will help to see you through.