Good question, even though no one ever answered your post. I know it is old and I’m sure you have either figured it out by now or moved on.
I’m posting an answer anyway since the discourse is searchable and people with the same question may find this for years to come. (just like I found it)
I wanted to know too, so I did some testing on this in the Mountain Flower Grove level where there are no enemies to mess with me. I am pretty darn sure that the answer is 30 meters, but if the paladin is beyond 30 meters when it gets the heal command it will try to move closer to its target all on its own until it is about 27 meters away and then cast heal.