A very interesting question. When I started with Fitness Training / Natural Bodybuildiung in 1987 I was told to train every bodypart twice a week. So I had a 3 day on / 1 day off / 3 day on workout routine. I did weightlifting 6 times a week and also some kick boxing sessions on top of it.
Needless to say that I gained weight and strenght fast, because I was full with Testosteron. And recovery was no problem with 19 years either.
Over the years I switched to a 4 days on / 1 day off programme, which worked very well for me. Since about 1 year I am using a 6 day on / 1 off programme. In other words I train every bodypart only once a week.
I was surprised to see some others in the web suggesting to do the same. For sure I need longer to recover, but would say that one training a week my body responds still very good to the work out. I have no idea if this is due to the long years of training and the experience or if it is a matter of age. Maybe both.
I found this video on YouTubem where another Natural Bodybuilder recommends also to train very muscle group only once a week. Have a look and let me know about your experience.