The answer depends on many variables, from training methods and calorie intake to the macronutrients provided by the foods you eat. You can lift weights all you want, but if you don’t eat enough protein and utilize the progressive overload principle, then you won’t be able to gain lean mass.
But even optimizing the hypertrophy process simply isn’t enough. Age, gender and genetics play their part in deciding how successful your muscle-building exploits will be.
There's no strict timeframe for how long it takes to build muscle. It relies heavily on the training regimen you follow, nutritional adherence, rest etc.
A 2018 study published in the European Journal of Applied Physiology explored the role of skeletal muscle damage and muscle protein synthesis. According to this study, muscle growth can be seen after about "10 sessions", but only after about "18 sessions" was significant muscle hypertrophy observed.
The study stated that increases in the size of the muscle in the early phase of resistance training (four or less sessions) is attributed to “muscle damage-induced swelling”. After 10 sessions, “a modest magnitude of muscle hypertrophy ensues,” and after around 18 workouts, "true muscle hypertrophy is observed”.
However, a 2017 research study also published in the European Journal of Applied Physiology concluded that "significant increases in lean mass” could be seen after just seven workout sessions, performed over the course of four weeks.
The study tasked 13 untrained men with performing dumbbell curls and shoulder presses twice per week for four weeks. They would complete 8 to 12 repetitions, working until they could not perform any more, and the weight was increased where appropriate as they progressed from session to session. Subjects also drank 500 milliliters of whole milk during training.
After the effort you put in to grow your hard-earned muscle, the last thing you want to consider is losing it (also known as atrophy). Unfortunately, a lack of muscle use, insufficient nutrient intake, or both can lead to that. Thankfully, though, it will take more than a missed gym session for this to occur.
Rates of atrophy typically vary depending on your current physical condition. The more "in shape" you are, the longer it will take to atrophy, and the slower the rate will be. However, typically after one week of little to no activity you can start to notice signs of atrophy.
A 2014 study into the impact of disuse on muscle size and strength - published in the Acta Physiologica journal – found that “even short periods of muscle disuse can cause substantial loss of skeletal muscle mass and strength”.
This does refer to complete inactivity of the muscle, though, with the 24 study participants subjected to either five or 14 days of on-legged knee immobilization using a full leg cast. After just five days, leg lean mass had decreased.
If you're trying to "bulk up", it’s important to know how to gain muscle in general.
When performing resistance training, small tears in the muscles are created by moving them through a range of motions under load. These tears are then repaired and built upon, using amino acids (protein) as building blocks.
In other words, muscle building relies on muscular effort (which is usually achieved through resistance or weight training, as well as adequate protein intake.
Targeted training
If you are looking to grow a specific muscle or group of muscles, also known as hypertrophy, you need to perform exercises that target them. The best approach is to remove the guesswork from your exercise program. You may think an exercise is focused on a specific muscle or group of muscles, but do you know for sure? Taking the time to do a little research into the exercises you are performing can make a big difference.
Leg Press
You may think your leg press routine is targeting your glutes, but the positioning of your feet may be targeting another muscle group altogether! If your feet are too low on the surface of the leg press, you’re actually working primarily on your quadriceps. Sliding your feet up will help readjust your focus on the intended muscle.
Load
In order to grow muscles, you must put them under the proper amount of load. If the weight you’re lifting, or the number of repetitions you are completing, does not challenge your muscles with the appropriate stimulus, they will not grow as efficiently.
When exercising for hypertrophy, generally you want to increase the weight and reduce the number of repetitions. When talking about the weight used to exercise, also known as the load, fitness professionals measure it as a percentage of your one-rep max.
For muscle growth, you want to train using 75% of your one-rep max. This is a resistance that should allow you to complete eight to 10 reps at a time. If you cannot complete at least eight, or if the weight feels too light, adjust accordingly.
Training at higher and lower loads can stimulate muscle growth, but it is generally accepted that this load and an eight to 12 rep range is optimal for hypertrophy.
An American College of Sports Medicine review states: “It is recommended that loads corresponding to one to 12 repetition maximum (RM) be used in periodized fashion with emphasis on the six to 12 RM zone using one-to-two-minute rest periods between sets at a moderate velocity. Higher volume, multiple-set programs are recommended for maximizing hypertrophy.”
Nutrition
Nutrition is a critical aspect of recovery and will play a big role in helping you build muscle. There are several factors that come into play, including your protein source, protein quantity, carbohydrate intake, etc.
"Muscle hypertrophy occurs when muscle protein synthesis exceeds muscle protein breakdown and results in positive net protein balance in cumulative periods", a 2019 study published in the International Journal of Environmental Research and Public Health says.
From the nutrition perspective, protein intake alongside resistance training is a potent stimulus for muscle protein synthesis. Information published by the American College of Sports Medicine adds: “People that exercise regularly also need to eat more protein than the recommended daily intake."
“To increase muscle mass in combination with physical activity, it is recommended that a person that lifts weights regularly or is training for a running or cycling event eat a range of 1.2 to 1.7grams of protein per kilogram of body weight per day, or 0.5 to 0.8 grams per pound of body weight.”
But additional research suggests this figure could be higher. A 2017 study in The Journal of Nutrition looking at the protein needs of young male bodybuilders found the estimated average requirement of protein was 1.7g per kilogram of bodyweight, and the upper limit was 2.2g per kilo of bodyweight.
As always, it is best to get most of the protein you need from whole food sources.