very subjective to the individual, but a VERY important topic to cover. I've been training for 12 years and one of things that keeps coming back time/time again is frequency for me. I like pmiller's numbering guide, though it can be difficult to figure out where you fall, but I overall agree.
On the standard split you hit a bodypart every 7 days. So if you hit legs monday, you'll hit em again the following monday. for me that doesn't work. Soreness lingers for different periods of time for all of my muscles, and if I don't hit that muscle immediately after it's not sore, then I make very little progress.
so if your chest takes 3 days to recover from soreness, then I'd really recommend you hitting it the 4th day, or even day 3 if the soreness is very residual.
there has been discussion of the actual DNA signaling from weight training, and they say that "signal" begins to wane after 72 hours. So some programs call for hitting the same bodypart within that 72 hours.
with that said, it's not so surprising that there are full body routines, half body routines, and even DC routines that are based around high frequency that work wonders for many people.
PS, one tip I've learned is this. If you are still sore after warming up, then don't do that body part. if you are no longer sore after a few warm up sets then you may be able to hit that bodypart again with great intensity.