Are you a person who is motivated to be productive measuring the amount you practice in hours? Perhaps you feel a sense of satisfaction in routine, but perhaps you might enjoy looking at your practice in terms of number of pieces explored, or number of light-bulbs lit. Personally, I feel a huge sense of accomplishment based on the breakthroughs I've had that day. If I've discovered a slight variation on my embouchure that allows a certain color or stumbled upon a new story-line for the big solo I'm working on, I feel that the amount of time I've devoted to practice that day is completely worth it.
The amount of time you need to practice is a question that only you can answer. If you have a huge competition coming up, multiple hours a day makes sense. If you want to improve over the course of the summer, 1 hour a day might be just what you need, depending on your specific goals. Keep in mind that balancing exploration and experimentation with fundamentals and repetition can help you "stay in shape" and encourage you to grow.
Become an expert on yourself. Know your learning style, your music preferences, your strengths and weaknesses. Use every resource at your disposal. Recordings, videos, books, websites, your teacher, other flutists, music stores - the number of people and places that can provide you with inspiration and ideas to enhance your practicing is unlimited. In fact, rather than asking the question, "How much should I practice," I would ask, "How much can I learn today about my flute, myself and my music?" Then, rather than "practicing," devote whatever time, energy and motivation you have to "discovering."