"How much should I practice?"
Well... how much time do you have? And how good do you want to be?
If you're a professional performer or aspire to be, you probably aren't even asking the question. For most professionals I know, the question doesn't even come up. They know it's every day, that usually means every spare moment. They can't help it, anyway.
But for the rest of us, the answer is tricky, and you have to take the two above questions and combine them. You see, everyone wants to be as good as they can be. But how much time do they realistically have available, and how can they maximize that time? For most adult learners, who are balancing their interest in music with their full-time jobs, their life partners, their children, and their passion for golf, the best answer to the question, "How much should I practice?" is this: As much as is appropriate for your life. If you've only got ten minutes, then do ten minutes. You can make it work.
At the very least, practice does need to be just about every day, because playing music is a physical activity that requires dexterity and flexibility, and though I haven't read any science on the matter, I believe that the mind/body connection--making sure your brain remembers the tunes and tells your fingers what to do, on time--requires daily exercise, too.
Practicing is very much the same as exercise, in terms of its time demands. How much should you exercise every day? If you wish to be a body builder, then three intensive hours a day or more makes sense. If you don't care about the poses, but want to look hot in red leather pants, then maybe two hours a day, including some heavy lifting? If you want to feel better, then an hour a day will do wonders. And if you just don't want to throw your back out when you're reaching across a table for the salt, then even ten minutes a day is more than enough.
In musical practice, you would be surprised at how much you can accomplish in ten minutes, if you focus on the right things. Yesterday, in a class at Bridgewater, Brassdoc said that he can accomplish much more in one hour of practice than he used to be able to accomplish in four, because he's learned what to focus on. He said, "Sometimes all you have to do is just correct three little notes, and a whole phrase comes together."
If you're really frustrated because you've been working on a tune and the same part keeps stopping you up... but you're also really crunched for time... then just work on that part for ten minutes. Get those four little notes at the end of the B part, and you may find that the rest of the tune falls into place. And when you have time during the rest of the day, visualize that phrase, sing it to yourself, and imagine your fingers playing the right notes. That counts, too.
*I think it's time to shorten my headers! We've already established that we're going 365 here, so let's just get to the point, it's day 209.