@vsw874 In my opinion, you are looking at this the wrong way. The problem is things like connective tissues, not mucle recovery. Connective tissue generally takes a pounding if you train with a high frequency, though admittedly some of this can be mitigated if you go easy on the per-session volume. Regardles, in my opinion the risk/reward ratio starts to become unfavorable with any frequencies over 2x/week in trained lifters. In my experience, 3x+ weekly frequencies are more likely to result in impingements and overuse injuries, especially as you begin to get older (30+)
Once you have recurring tendonitis or serious shoulder problems you are seriously fucked. This not only impacts your ability to train hard in the gym, it impacts many things in your life right down to playing with your kids, sleeping properly, etc.
If you are not a serious competitive bodybuilder, I think it's best to train in a sustainable manner which keeps you injury free. Splits such as upper/lower do a great job of limiting elbow and shoulder usage (all pushing and pulling goes through the shoulder girdle). This is my personal opinion, but I would limit my shoulder and elbow usage to 3 days per week maximum as a very general principle. If you are looking for a good source on how to structure things for longevity I would recommend joining Lyle McDonalds facebook group. There are plenty of people there that can talk about the theory around structuring a sustainable split, along with problems they ran into with certain splits and frequencies. For example, quite a few people have noted that doing a PPL 2x/week where you train push and pull consecutively can lead to shoulder problems. Too many people just throw together a split without thinking of factors like this.
I'm not suggesting you WILL get hurt or injured when training with higher frequencies, it is dependent on age, genetics, total weekly volume, etc. Just that it is more of a risk, in my experience.