Xanax is metabolized by CYP3a4
Tobacco
induces enzymes CYP1a2 and CYP2b6, and long term/heavy smoking
inhibits CYP3a1 and has been speculated to also inhibit CYP3a4
Inhibition of CYP3a4 would actually lead to increased effectiveness and duration of the xanax, it wouldn't reduce it.
Although, to answer your question regardless of xanax, maximum liver
induction by tobacco has been shown to occur at 12 cigs per day. I am unsure how much tobacco it takes to maximally inhibit CYP3a4, but likely a similar amount.
But again, if anything tobacco should lead to an increase in xanax effects, not reduce them. It's likely a small or negligible effect, though.
I do encourage you to reduce your smoking, anyways
