All of them. Every workout. But I'm weird like that.
I mean like say squating once a week or say 2 to 3 times a week. or benching once a week or 2-3 times a week?
All the latest studies say it's best to train multiple muscles in a workout. For instance, if you're done chest, also do tris!
Think training's hard,. try losing!
I've had some success by mixing it up. For instance, I may do a workout routine such as this for a while:
Monday - Chest/Shoulders/Triceps
Tuesday - Back/Biceps/Legs
Wednesday - Off
Thursday - Shoulders/Chest/Triceps
Friday - Legs/Back/Biceps
Saturday and Sunday - Off
The first muscle group on any given day gets hit the hardest, but since I'd be training each major muscle group twice per week, I stick with low volumes, such as 8 total sets per larger muscle group at the most. So for Monday, I may do 2 exercises for chest (8 total sets), 2 exercises for shoulders (6 total sets) and then maybe 3-4 total sets for direct triceps work. I follow a similar formula for each day. I'd do calves and abs whenever they feel fresh enough. If my training intensity starts go decline in any given area, usually I take it as a sign that I'm doing too much. If I notice that I'm progressing, then I stick with that works.
After a while, like maybe 2 months, after a short break I'll switch up my routine. For instance, I've been on the following routine for about two weeks and it's been a good shock for my body:
Sunday - Chest/Calves/Abs
Monday - Back/Forearms
Tuesday - Shoulders/Calves/Abs
Wednesday - Legs
Thursday - Arms/Calves/Abs
Unless my forearms or calves feel fresh, I won't work them. Especially forearms. I still don't do a lot of direct work for arms since they already get a lot of work. For instance, on Thursdays I may do 2 isolation exercises for triceps (extensions, pushdowns) and for biceps I'm fond of doing multi-exercise sets. That is, I'll do maybe 4-5 total sets for biceps, but do a variety of exercises, after warmup.
Anyway, this works fairly well for me at the moment. I recommend really paying attention to the signals your body gives you so that you avoid overtraining. If you feel progressively sluggish/tired/fatigued/weaker, then you're probably doing too much.
Do some researching on the CNS and how it works relative to bodybuilding training.
I agree with the overtraining part for sure. This especially comes into play when dieting.
I'm a firm believer that you should do as much as you can - as often as you can - without overtraining. Most of the time that is 4 days on 3 off for the natural lifter. Add in cardio and reduced cals(dieting) and it becomes even more of a tightrope walk.
Watch yourself for the signs of overtraining.
Bulking you may be able to hit at least certain groups(lagging groups) twice a week. Cutting things will change and you have to follow your body. I am on CKD(my own version) right now, and recovery times are pretty compromised. Im playing it by ear at this point. Heavy days for sure on Carb up days and the day after.
Say you hit your legs hard and you're sore for a few days.. "Monday- legs Tuesday-Off Wednesday- Arms" So say its Wednesday now and its time to do some upper body work but your legs are still sore.. Would it be a good idea to give it another day or would it not matter seeing as how you're lifting with muscles that are not sore. Or could it be your CNS isn't fully healed from the leg workout so you're probably not going to see your best results in the gym this day so you better give it another day if you want to hit your arms right. Know what I mean?
Additionally, soreness is no indicator of whether or not it's time to work a muscle group. Active recovery is a very beneficial technique to speed recovery and reduce soreness - it involves light work for a muscle group recently worked for the purpose of increasing blood flow and stretching the muscle.
Furthermore, DOMS is reduced with increased frequency of working a body part, provided you aren't over doing it.