For most people, most of the time, muscles will be fully recovered and ready to be trained again in two days. That’s right, muscle will be ready to train again in two days, which means if you wait any longer than this to hit that muscle again, you’re losing ground.
It’s not that “bro splits” (where you train each muscle group hard once per week) don’t work, it’s that they don’t work as well as other approaches, at least for most people.
Now sure, if you happen to be Big Ramy or some other similarly huge lifter who regularly squats in the 700-800 range and benches north of 500, okay, maybe it takes your muscles a week to recover. The rest of us only need a day or two until we’re ready to hit that muscle again, and if we don’t, we’ll start losing our previously-acquired gains.
Think of it this way: The bigger a muscle is, the more damage it sustains during training and the longer it’ll take to recover from that damage. You probably already know intuitively that your biceps recover faster than your hamstrings, for example.
That being the case, most guys should train their bigger muscles (legs, back, and chest) 2-3 times per week, and their smaller muscles (pretty much everything else) 3-4 times per week.
This reality is the rationale behind my recommendation for 3 “whole body” workouts per week for most lifters. The best approach involves doing a squat, push, hinge, and pull movement, along with 1-2 optional movement for smaller muscles such as arms and calves, each workout.