While one leg day a week is better than nothing and can help improve strength to some degree, generally speaking, in order to see strength and muscle gains, it's best to target all your major muscle groups at least twice a week. For instance, a 2015 study published in The Journal of Strength and Conditioning Research found that a group of participants who trained three times per week with the same total load as a group who trained only one time a week saw greater gains in muscle mass than the once-per-week group.