I strongly believe that exercise is relative to everybody based on what their body is telling them. A simple blanket statement such as "you must train at least 4 times per week or you aren't getting any benefit" is ludicrous and short sighted. At the same time I love giving you guys starting points as long as you know that they are just that. Starting points. Having complete dependency on somebody denies you the opportunity to achieve your results and maintain that progress permanently. Here are some starting points and guidelines to start with when figuring out how often you should exercise.
When first starting an exercise routine or just getting back into one, you need to figure out how long it takes you to regain your "optimal condition". Your optimal condition is the state in which your energy is at full capacity, your sleeping patterns are benefiting you, your body is functioning high without extreme muscle fatigue or soreness and your hunger and cravings are in check. If one of these areas is off and is costing you in your day to day life then your exercise routine needs adjustment. This can mean either the intensity, duration, type or frequency of exercise.
If you are someone who is just starting out and has never exercised before I would recommend 2-3 times per week for roughly 30-60 minutes of some type of resistance training (body weight or dumb bell type training for example).
If you are someone who has been exercising routinely but wants to switch things up and try to work out more frequently then by all means go ahead. I train people who work out 5-6 days/ week in some capacity. Meaning that they aren't necessarily throwing tires around and sprinting up mountains for 5 hours a day 6 days a week but they are doing exercise that benefits them and their goals while maintaining communication with what their body is physically telling them. Remember, you want to exercise and put as much beneficial stress on your body while maintaining your optimal condition.
With that said there is nothing wrong with working out and getting some muscle soreness afterwards. Soreness is awesome, but too much soreness and fatigue that causes you to not be able to maintain your routine and physique is not what we are aiming for.
Generally, people who start an exercise routine are able to dial in and find the perfect blend of rest and exercise in about 2-3 weeks. Your body adapts to the external stress that you intentionally put on it
(exercise) and all of a sudden you find yourself in a weird spot. You start to crave it and want more. Now your stamina increases, your clothes fit a bit differently, your energy goes through the roof and you are now someone who exercises.