Resistance forces growth and the compounded effect that occurs over the course of many months and years is what you are working towards. The challenge of being able to convince yourself that enduring pain and putting your body through physical challenges that it would rather not do, isn't always easy nor is it something that we always have time to do. Finding that perfect blend between days when you train and days when you don't so that you can still accumulate a significant amount of gains, is usually the struggle I see most when offering advice to those still trying to find this balance. So the question then remains; how often do you have to train for results? While the answer to that question will vary from individual to individual based upon a few factors such as genetics, one's ability to recover and how much stimulus is actually needed to induce hypertrophy, there are a few training splits that you may want to experiment with until you find your sweet spot for growth.
3 Days a Week
At the bare minimum, if you are looking to amass any sort of muscle, my personal opinion is that you need to train at least three days a week. The reason for this is very simple in that you can put together an extremely productive program for yourself that addresses groups of muscles that perform the same tasks so that you can achieve a lot within one workout. The three day plan I'm referring to is the PUSH/PULL/LOWER BODY training split that will see you train your entire body over the course of those three days. Group your CHEST, SHOULDERS and TRICEPS for the first workout, your BACK, TRAPS and BICEPS for the second and your QUADS, HAMSTRINGS and CALVES for the third. Give a day's rest in between each workout with a full two days off after the last. As long as you go as hard as you can on those three days, you'll have sufficient time to rest and recover on your days off and while having lots of time to attend to the other things you have going on in your life.
4 Days a Week
The four day training split was very common back in the mid to late 90's and into the early part of 2000. This may have had a lot to do with the fact that the best bodybuilder in the world at the time followed this split, but regardless, many of us followed it and did get great results from it. The idea was that you would pair up two different muscle groups (that also worked in synergy with one another) and hit those muscle groups once per week. You would train the larger of the two first followed by the smaller muscle and you would set your training split up so that you trained for two days in a row, take a day off, train another two in a row and then take two days off. So it would look something like this; CHEST and TRICEPS, BACK and BICEPS, SHOULDERS and TRAPS and then LEGS. The workout that was followed by the two day rest would be for your lagging muscle groups so that they had more time to heal, recover and ultimately grow.
5 Days a Week
Some of us are so resistant to the training stimulus that we have to beat our body into submission week after week for it to grow. If you fall into this category, you know how frustrating it can be and you also know that you are going to have to spend a lot of time in the gym just to see a hint of new muscle. The five days a week training program is probably what's going to work best for you which will be perfect if you just love to train anyways. Set your week up so that you are hitting CHEST on one day, BACK on another, LEGS in the middle, SHOULDERS later on in the week and then finish up with ARMS. You can do this by training five days in a row straight through the week and then take the weekends off or you can just insert a day off during the split whenever you feel you need it or can't make it to the gym on that day for whatever reason. The nice thing about this training split is even though you'll be in the gym more often, you won't be there for as long on most days because on three of the five days you're only focusing on one muscle group versus two or three.
The only way to really tell if what you are doing is enough or too little is by giving the training split you've decided to go with a chance to work or not work. Most times people actually do too much and way more than need be for optimal growth while other times people think that what they're doing is sufficient when in reality it's not even close. Similar to when you're thinking about ordering another pair of Super Freaks or Super Shifts and you say to yourself, "nah, I already have a pair" when deep down you know you can never have too many pairs of Heydays. A true reflection upon what you're doing and what you've accomplished will reveal the right answers for you and once this occurs, you'll know how often you have to train to produce results.