From what I've heard tolerance/lack of effectiveness can occur with many meds. My allergy doc has me go off/switch meds every so often for 6 weeks and then I can go back to the other one. Apparently the body becomes so acclimated to whatever med is used regularly that it doesn't work as well.
The 6 week time frame is the only one I can provide, as that was the case with my allergy med. I don't know if that would be the same for H2 blockers.
The same thing happens with albuterol inhalers...the more they is used, the less effective they are. Before my Nissen surgery my asthma doc had me stop the albuterol, so it would be able to work during and after my surgery if necessary.
Sorry I can't be specific about H2 blockers, but this is the best I have to offer!
Good luck with your quest!