Originally Posted by Magumi
Rennaissance Periodization did a great job of sumarizing hypertrophy landmarks recently; these included minimum volumes required to maintain a muscle size, split by body part. From memory, for experienced lifters, most body parts require at least 8 (hypertrophy-oriented) work sets per week to maintain, the exceptions being 4 weekly sets for triceps and biceps (because you use arms in virtually all other exercises anyway) and 4 weekly sets for harmstrings (because harmstrings tend to be more fast-twitch dominant for most people, and can therefore handle more weight at the expense of lower volume endurance).
(Personally, if I were to design a minimum maintenance programme, I would prefer doing two sessions per week).
I read through that series a while back. He does have a lot of good ideas that I agree with. Among them: counting number of hard sets in a moderate repetition range as the primary way to measure volume, accounting for overlap in a logical way, advocating different frequencies and volumes depending on the size and specifics of particular muscle groups, and some others that I'm forgetting at the moment.
That said, I think his volume landmarks are unreasonably high pretty much across the board. Even if you go to 4-5x/week training and keep some muscle groups at the (relatively high) maintenance recommendations, it's still a really large number of sets. The average person will struggle to recover from the workload.