Interesting topic...
I started running about 3 years ago, initially to lose some weight I had packed on since college.
I eventually got to the point where I could run "indefinitely"... "indefinitely" meaning I would likely pass out from bordom before passing out from fatigue.
At the peak of this routine, I was running just under 5 miles 6 days a week. To be precise, I was running 28.5 miles per week.
Like someone else mentioned, I tended to get sick more (got two different flus in two months), a constant cold, and very sore tendons in one foot and one knee. I was so obsessed with it though that I just went to the doc and he gave me some hefty anti-inflammatories and I just kept pushing forward.
I finally started to get burned out, thankfully before I permanently or severely injured myself. Right now I run about 3 miles 4 times a week, sometimes 5 if I feel like it.
Anywho, unless you have specific goals, there's no reason to beat the hell out of your body. I recall feeling my best when I was running about 20 miles a week, just like what someone else said. When the winter gets here, I'll probably go back to that. But here are a few tips:
1. Don't do so much that you'll get burned out. Running 15 miles a week over several years (preferrably a lifetime) is much better than running 30 miles a week for a few months and getting too burned out to continue.
2. Someone else brought up "eating your workout". If you're trying to lose weight, this is very important; but if you're in good shape, make sure you eat enough food (and the right kinds)... otherwise your body will deteriorate.
3. GET ENOUGH SLEEP. I have insomnia, so believe me when I tell you that not getting enough sleep is probably the easiest way to wreck your body, especially if you have an excersise regimen. Do whatever it takes to get your 8 hours (or whatever the key number is for you).
4. Be consistent. It's much easier to keep up with your program if it because part of your daily life (like brushing your teeth... hopefully). I get home, screw off for 45 minutes or so, and then I run.
The best advice is just to do what works for you, and unfortunately, that's just simply a learning process. No one can tell you how to get your own best results or what your optimum frequency and intensity is. You'll have to figure that out on your own.