Use a permanent variable instead of a temporary one, i.e. healtimer instead of @healtimer. The reason most people use temporary timers is because (1) it's usually a short period of time so relogs wouldn't matter, and (2) there's a limit to how many permanent character variables you can set. To work around (2), you can have the NPC delete the variable after healing:
// heal, etc.
set healtimer,0;