Also bear in mind that the 90-day QT period restarts when there is a change. So, if a snake in QT pops with an RI then the QT clock doesn't restart until after the snake has been through the course of antibiotics and stopped showing symptoms.
Bringing in new snakes restarts the QT clock for all snakes in QT if you have to run them in close proximity. Personally I don't buy more snakes if there is one in QT already.