The tipping point for me is when weed takes priority over more important things. For example, if you blew your rent money on weed, then I would say you had a problem. If weed is just one facet of your life and you "want it" rather than "need it" then I think you are OK (obviously that doesn't apply to medical users).

As for your lungs, if you are really concerned it might be an idea to invest in a high quality vapouriser or start cooking/making drinks with weed.