The lads over at Primer Magazine have done any sartorial novice a great service by having a great example of exactly where ones suit sleeve should end and shirt cuff begin. Too often the suit sleeve ends exactly where the shirt’s cuff does as well, making it look as if you don’t know how to dress yourself. This should save you any embarrassment. Between a 1/4” and a 1/2” should be showing for the record. While we’re on the topic, only a long sleeve shirt should be worn with a suit precisely so you can achieve this look.