It's limited only in how fast it works. But As long as Manta is trying to keep his distance, Reaper can go all day. Just pop up next to him shoot away, back away heal up while keeping Manta At a distance and than pop back up next to him.
His regeneration is in the fact the his body is in a constant phase of dying off and rehealing itself, so it's not the best healing factor and it's not very fast either. If he gets a fatal injury, he would still die.