Found the rule. Section 3 of the NFL rule book. Five yards isn’t specifically mentioned. But there is obvious leeway given to the judgement of the covering official.
Item 2. Illegally Downfield. An ineligible offensive player is illegally downfield if:
(a) he moves more than one yard beyond the line of scrimmage without contacting an opponent
(b) after losing contact with an opponent within one yard of the line of scrimmage, he advances more than one yard beyond the
line of scrimmage
(c) after losing contact with an opponent more than one yard beyond the line of scrimmage, he continues to move toward his
opponent’s goal line.