Both my older sister and I are, uh, drug-hobbyists... but my mother approached each one of us quite differently upon finding out.
With my sister, who discovered her interest in drugs before my own and was sort of our family's first real case of substance-abuse, my mother kicked her out of the house, demanded that she get clean, go to rehab, etc.
But with me, my mother is taking a somewhat different approach; she knows that I use, but she hasn't demanded that I check myself into rehab, hasn't threatened to kick me out of the house or anything like that.
I'm seeing a therapist, and I'm working on the whole substance-abuse thing slowly but surely; my mother knows that much, and I think that, at this point, she's OK with seeing how this all goes before taking more extreme measures. I think we all know that this is unsustainable, and I'll need to clean it up sooner or later. I've got greater dreams and aspirations in life greater than and beyond merely getting high all of the time, and in order to pursue these things, well, the drugs will need to go... (shrugs) and so we'll see what happens.
EDIT: If I were told to shape up or ship out right now, at this very moment, well, I'm not sure how I'd respond... I think that I'd probably pack up and strike out on my own for a bit before committing to rehab or anything like that. I just haven't yet reached that level of commitment...