It's 20 days, but the math is not quite that straight forward. They need to be on an active minor league schedule for 20 days. Which can correspond to 21 or 22 or 23 major league days, depending on your particular major league schedule.
My rule of thumb, because it takes all the math out of play . . . if your minor league season starts on the AM cycle on Wednesday (for example), then do your promotions after the AM cycle on the following Wednesday.