exercise per week?

My friend who works in a gym recommends three to four times a week.

Depends on your goal. Per the source article, weekly exercise recommendations for general fitness are:

" Get at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous aerobic activity a week, or a combination of moderate and vigorous activity. The guidelines suggest that you spread out this exercise during the course of a week." and "Do strength training exercises for all major muscle groups at least two times a week. Aim to do a single set of each exercise, using a weight or resistance level heavy enough to tire your muscles after about 12 to 15 repetitions"

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If you want to train for a sport or event, you will probably want to do much more than the minimum recommended. I try to workout at least 5x a week, with 3 of those days being running (or some other cardio) and 2 of strength training. My workouts are typically 50 mins or more, but sometimes as short as 30 mins if that's all the time I have.

You should be active everyday for at least 60 minutes, whether its walking, running, biking, stretching, etc. The rest will vary depending on your goals.

at least 5 times per week........