In reply to climbercool:
Google 'training supercompensation' and have a look at the images, there is a pretty standard graph that demonstrates how your muscles recover from training, as far as I understand it, it's pretty well researched and agreed to be correct.
Basically if you push your muscles too soon again after exercising, you will compound the damage done to them, making them steadily weaker (and more susceptible to injury). Equally if you leave it too long to exercise the muscles again you will lose any gains made from the first session. There is a prime period somewhere in between when you should exercise for maximum benefit.
This rule applies seperately to individual muscles or muscle groups so you can train on consecutive days as long as you work different muscle groups each day.
In response to the OP, if it were completely under my control, I would never rest longer than 2 days between sessions, it's just the intensity that might vary, but then I enjoy the act of training as much as the improvements that it brings. Unfortunately life and injury doesn't always let that happen.