I like to keep mine separate at least a month. Others may do it for more or less time. I choose that long because if the plant was sprayed for pests right before it was put up for sale, it may take a few weeks for eggs to hatch and larvae develop into a recognizable pest. I found that a month wasn't even long enough once to find out that the plant had scale. But it sort of depends on how the plant looks and where I got it as to my isolation rules. But looks can be deceiving too! The healthiest plant from the best source can have a surprise for you!
I just bought 2 new orchids last week from a local grower during a visit to the Atlanta Botanical Gardens and right now they are taking up the bathroom sink. Of course my boyfriend is not too happy about that-more because he's afraid he's going to knock them over.
well for as long as you can...........and make sure you thoroughly spray the orchids with a general insecticide too.
( Garden Safe is good )
I say this from experience - having battled with mealy bug and mite infestations, that spread from room to room, from plants that I thought were clear.