An RI can be contagious. They actually recommend you clean the tank after a ball gets over it as it could catch it again. Quarantine is needed anytime you bring another snake home to protect the others in the house. I do one month minimum after the vet has given them a clean bill of health including poop test. If you don't do the vet I would make it a minimum of 3 months at least. RI is a life threatening infection to snakes, it can and probably will kill them if it goes untreated. We had one that came in with an RI and we had to give it daily injections for 7 days after the vet did the initial for a total of 8 days. She went to her follow up and the vet believes the RI is gone, but now she has extra syliva according to the vet. The vet wants to see her again on the 24th and if the extra syliva is still there he plans to do a culture. Trust your vet and not some syco at a pet store with a high school diploma or worse!