I would go with at least 4 weeks. My thinking (and I use the term loosely) is this. Lets assume as soon as you bought the fish the ick parasite if that’s what we are talking about dropped off the fish now you don’t see any signs of infestation but it some how came along for the ride with the water in your bag it has 18 hrs to settle to something in your tank (QT) I hope to reproduce. The reproductive stage can run from 3 to 28 days. Then it evolves into the free swimming stage and its able to survive from 24 to 48 hrs while looking for a host. If you add all those times up ( I don’t have enough fingers and toes so you will have to do it) but I think you want to wait at least 4 weeks to be sure