Depends on the boutique and the relationship with the SA. If you come in and request a B/K right away then you will most certainly wait long time. If you also purchase other items, time gets shorter. You get the idea...
Also, I waited a LONG time for black. My particular boutique has received a few black epsom B30 over the past year or so but none in togo. I wanted togo. While I was offered other quota bags each semester ... I think I still waited over a year for black togo B30. Soooooo it depends.... but some boutiques get large shipments and have black more readily available... especially if you're not as particular as I am about size, leather AND hardware. Good luck and happy hunting!