sorry if I didn't read the entire thread (just don't have the time), but here's my two cents:
I've been working on rehab clinics for a couple years now (I'm a psychologist to be) and I know many people who are a lot into drug (both patients there and friends in general).
Don't know how it works for you, but I can't stand my pothead friends when they're under the effect. You said she gets high pretty much all the time, except when she's with you. Well, I guess that if she keeps it that way, you'd be fine, except that saying that is pretty much saying "it's me or the drug", which can't be good for the relationship.
I don't want to demonize the use of drugs, I won't show up and say "hey, if people do drugs they start stealing shit and selling their bodies" because that's not true. However, it's undeniable that pot makes you dumber over time, makes you just want to talk about it, just want to hang out with friends that do it (claiming that pot is a "culture" is just an excuse to just talk about it).
I'd guess that for a constant user of a drug to be in a relationship with somebody that doesn't use it, the sober one should be 100% laid back and not care at all about the use, which doesn't seem to be the case (were it the case, you wouldn't be asking for opinions, I guess). I honestly can't see this relationship going much further but hey, you guys should go ahead and try to prove me wrong. At least have some fun while attempting.
Ultimately, I think you should just lay back for a moment and see where this goes. If you can still stand her behavior, but you're annoyed by what might happen if she keeps doing it or something, try to work it out for now. Have the relationship end when it has to, and not because you think that someday it will have to.