or simply scrap the time out altogether, and implement, "goodbye [npc name]" as the the way to end the session with the npc. granted there should be a fallback timeout in case the use forgets to end the session, but that could be based on the distance between the user and npc, (if they ran away wthout ending, take the difference of postions, if they are over a certain radius, session auto ends) or say a 3 minute timeout (in case they don't move, but forget to end the session) 3 minutes should be plenty of time, and the number scould be adjusted anyway.
and I'd say more like its 10,000-30,000 at least. 1,000 will only get you to Asia.