The speed of regenerating hearts when a player has regeneration effect does not depend on the level of the effect, but it depends on the frequence of the player receiving the effect.

For instance, you enter "/effect @a regeration 99999 <any number between 0~255>". The speed of your hearts regenerating is the same. But when you enter "/effect @a regeration 99999 0" into an active repeating command block, your hearts will regenerate like a flash.