How often you should weight train ultimately comes down to your goals, and your adherence. I can lay out the ideal workout program but if it’s a frequency you can’t realistically sustain, then it’s pointless.
Generally speaking, if you are new to weightlifting I recommend starting at 2-3 days a week. Believe it or not, even as little as two days a week is more than enough stimulation to trigger muscle growth. Think of it this way. When you start drinking, you don’t need 15 shots to get you tipsy. The novelty is so new that you only need the minimum effective dose to elicit the most amount of change. Two full body workouts will hit the entire body, and produce the starting muscle growth you need. If anything, doing more than this may actually risk you overtraining and delaying the maximum amount of muscle you can grow. Remember, more isn’t better. You want enough stimulation to break down the muscle, ALONG with the downtime to allow for recovery and muscle growth to actually occur.
As you progress more and more into your lifting career, you may start to find that your progression in the gym stalls, or that you aren’t putting on as much size. Only then, should you consider adding more days or more sets to help elicit more growth. Going back to the alcohol analogy, the version of you who's been drinking for 5 years will need more shots to get even a little buzz compared to the version of you who just started. It is not about adding volume for the sake of volume. When you see these pro level bodybuilders doing tons of sets it is because they have accrued so much muscle and spent so many years training, that they actually need at MINIMUM that many sets per muscle, just to elicit even the slightest bit of growth.
To give some guidelines:
Novice - 8-10 weekly sets per muscle
Intermediate- 10-15
Advanced - 15+ sets
So again, only add more sets to your workout when you find the current volume is holding you BACK!
Which brings us back to frequency. How many days you should workout comes down to how many sets per muscle you need based on your experience level. An advanced lifter will need more days to spread those 15+ sets versus the novice only doing 8 sets. So the next question you have must be “how do I know how many days to split it up over?”
The research shows we can maximize the muscle growth stimulus from anywhere between 2-10 sets per muscle in a given workout. So yes, if you are a novice trainer you could do a body part split. However, I still prefer recommending a full body split because doing all your sets for one muscle gets pretty exhausting by the third exercise. Instead of squeezing as much as you can in one day, why not split those 10 sets over 2-3 days? It’ll also allow you to come at those later exercises more fresh which in turn, allows you to push heavier weight than if you just tagged it on at the end of a chest day. Research also shows the muscle building signal after exercise goes back down to baseline after 48-72 hours, so you also get the benefit of re-elevating that signal rather than waiting 7 days to hit it again.
My final point comes down to accountability and sustainability. This again, is why I like full body splits. Life gets in the way. If you miss your first workout that is totally fine because you’ll still hit the entire body two more times later in the week. If you do body part splits, and you missed your chest day, now you have to wait a whole week before you can hit it again. It’ll risk totally throwing off your entire routine. So, I’m not against different types of splits. I just highly suggest choosing whatever split you know will produce the most results based on adherence. If you know you have a busy schedule then try a full body split. If you know you can stick to your routine and really want to try a body part split or an upper lower split then do that. The difference in gains here is minimal when all else is equal.
