I usually keep my new orchids in another room on a window sill for about 3-4weeks. Just to be safe and help them become adjusted to their new growing environment. I also want to make sure that even though I cleaned the plant and put it in new media there are no little monster eggs attached to the roots that I cant see.
The more completely you can isolate the better. So if you do have a separate room they can stay in for the nonce, that will be better than being in the same room. If that is not an option, then a separate shelf is better than being on the same one.
Keep in mind, just because you did not see any pests, that does not mean they aren't there. Some are well nigh undetectable as larvae/nymphs. Furthermore, there is the possibility of unhatched eggs being present somewhere on the plant or in the media.
Duration, again, depends somewhat on what you can practically do. Generally, 3-4 weeks should be more than long enough for any unhatched eggs (should any exist) to do so and for any larvae/nymphs to reach a visible size. If you have ever noticed when reading a pesticide label for plants, retreatment of the plant(s) is usually recommended at two week intervals. This is based on the hatching timetable the eggs of many pests follow.
I keep my new orchids isolated for about three or four weeks in a different room and, if I don't quite trust where it came from, I keep it isolated longer, until it shows signs of thriving (to help protect the collection from virus--not a sure bet but it is just another measure I like to take).