Its not just about body part recovery, the main problem is with systemic recovery - i.e. how your entire body recovers from a depleting workout in terms of fuel stores, electrolytes, and the CNS.
Not only do you want to let the specific muscle and connective tissues of each bodypart recover, also leave enough general recovery to make sure you aren't overtraining.
For example training 5 days in a row, but different bodyparts, may be enough for local recovery because you're leaving about a week in between training each particular workout, but if you're doing any kind of intensity those 5 days in a row will be severely taxing in a non-specific, systemic sense. 2 days recovery will probably not be enough for that, and is a major reason why people tend to get stale near the end of a program.
As a rule of thumb i try to have about as many rest days as i have workout days in any particular time-frame. So if you're working out 3-4 times a week, have 3-4 rest days aswell.
Quality > quantity.
Do you have any idea how many times I've tried to rep you, but it won't allow me until I "spread some rep" around?! haha
I agree with Gaz -- because the body consists of more than just muscles, you have to consider your body as a syncronized unit -- a system. This is why I dropped "muscle group" training last fall and saw the light that is the upper/lower push/pull system. Once you understand and appreciate how the body works together not just with muscles but tendons, joints, ligaments, blood vessels, lungs, organs, skin, hair, nerves, motor units, MIND, hell everything! It's all involved! Once you grasp this synergy as an element of your bodybuilding than you see rest in a whole new way.
I use to also think "okay I need X number of days until I can train my chest again", but there are so many more variables to factor in to the equation.
There are all sorts of little things that can also affect rest in a positive way. Never underestimate the power of your mind. Stress is one of the most counter-productive elements to your training (that asshole, cortisol!), and so learning how to reduce stress in your life is crucial to proper rest (it's not just a matter of not bench pressing for 4-5 days). I'll admit, I intermittently use marijunana as a stress reliever. I'm not talking 5 joints a day. I mean (I use to smoke semi-regularly for not too long before I started lifting...never even close to a "stoner") I'll not do it for a month or so and then if something comes up that's stressing me out I'll induldge in a couple bowls one night...maybe even 2-3 nights in a row if it's bad.
I may get grief for this, but hey....I was a marathon runner, and from that as well as my experience lifting I've learned how to listen to my body. And, I've learned to weigh the good vs the bad. The marijuana in small occassional doses clears my mind and prevents that cortisol from releasing. For intance, when I first started lifting I underwent a harsh breakup, and was nearly depressed for 2 months. My BF went up noticeable and my lifting progress dropped. STRESS! It screwed me over. This is why I'll tell people who are dieting and tell me "I'm usually good but I'm craving a candy bar" that well if you're consistent you might as well eat the bar. What's worse? One 300 calorie, 30g sugar, 10g sat, 3g trans fat snack? Or, a week of stressing out over it?
Weighing the good vs the bad = emotional and physical health.
I may occassionally smoke some weed, but I also lift compounds for every plane of motion consistently and intelligently 3 times a week, I run, I eat 6-7 small meals a day with well-balanced macros, I rest well, I stretch, I research psychology to employ useful mind-tricks to make my body work the way I want it to, I drink lots of water, and I'm generally very active.
Some may think my methods are controversial, but I just added 20 lbs to my 10-rep squat 2 nights ago and that was the day after running 6km at a decent pace (normally I run 3.7km @ a 3:45/minute km...pretty good).
See what I mean? There's so much more to rest. It's not just refraining from whatever the activity is. It's everything you do to/with your body. Synergy is key.