I think most people just go with what makes them most comfortable. I QT for 6 weeks in a different room, assuming everything appears normal with the new snake. If it was a rescue, I would do a full 90 days of healthiness before considering him/her problem-free. QT isn't just about what the animal brings in with it. I have had one BP get a mild RI weeks after I brought her into QT - too long for it to have been the "fault" of the breeder I got her from, but none of my collection was sick. Long story short, I believe the stress of moving left her open to a mild RI, which I treated with no issue. Keeping her in QT for longer than the couple of weeks allowed me to get her settled in and fully healthy, even though neither she or my collection were the direct source of her illness (she stayed in QT an additional 30 days after all symptoms cleared).
Just to be clear - If the snake has any issues, mites, etc., the clock restarts AFTER they are back to full health.