As with most aspects of fitness, the answer to this question is not straightforward.
If you want the simple answer (and to not read this post) generally 3 times per week is the minimum you need to accomplish any fitness goal with 4-5 days being optimal.
The longer answer is a bit more involved, but may be worth your time.
Before you determine how often you need to workout, first pick a goal. What you want to accomplish determines your course of action. You need to have an idea of what you want through working out before the answer can be responded to correctly. For example, if you want to run a half of full marathon, you may need to run 5-6 days per week depending on when you started training and how far you are form your race. If you want to be healthy, the above 3 days per week is ok. If you have extreme body recomposition goals such as droping 30 pounds, you may need to work out between 4-6 days.
Limitations, we all have them and they need to be taken into account. Limitations range from physical such as a broken bones or lacking strength to lacking skills such as not being able to squat properly. Sometimes, you may need to work on your limitations before you can start your exercise program. For example, if you weak glutes cause you to have lower back issues, you may need to work on strengthening your glutes daily until the problem goes away. Having back issues will directly impact how often you can work out. So while you may work daily to learn to activate your glutes, you may only actually workout 2-3 days per week. Your goals will determine what you want to accomplish but your limitations determine where you begin.
Type of Exercise
The type of exercise you do will also effect how often you do it. If you want to build strength, 3-4 workouts per week with at least one day rest between may be best. Those looking to build total body muscle (muscular hypertrophy) may workout 5-6 days and work on different muscle groups every day (Chest Day, Back Day, Leg, Day, Arm Day, etc) with one day off per week.
Something important thing to consider when deciding the amount of days per week you workout is rest. Working out puts stress on the body, weight training especially causes micro tears in the muscle which lead to inflammation. The body then repairs these muscles and adaptations occur that make you stronger, over time this change is seen externally. However, if you do not allow the body to heal, these micro tears can occur too often and may prevent you from seeing results. Taking that into account, take 1-2 days off per week minimum and always start slow when beginning a new exercise program.
Generally follow these tips to avoid overtraining:
- Make sure your nutrition is up to par (see a dietitian!)
- Get 6-8 hours sleep per night
- Take 1-2 days off per week from working out
- Drink plenty of water
- Never work out sore muscles
- Listen to your body, if you feel tired, rest.