You have to do weights at least three days a week. Research says that, at a minimum, you need to train a minimum of two days a week to maximize muscle growth. If you want to make the biggest gains, you should train each muscle group twice a week, according to a new review in Sports Medicine magazine. When it comes to the question of how many days a week you should train, that really depends on your goals.
Three to five days is a good number (as long as you get enough rest between sessions, more on that later). I usually do two days for the upper body and three days for the lower body per week. If you train less than four times a week, a full body split (that is, following full-body workouts most days rather than splitting it) is probably a better approach.