If its just a flameout addition I add them as soon as the boil stops then start chilling. But I use a cold water bath, so YMMV with a chiller.
A hop stand is when you let your flameout hops steep hot before starting your chilling. It can add a significant amount of IBUs, since alpha acids will continue to isomerize (albeit at a slower rate than at full boil). I generally stand for 30-90 minutes for hoppy styles. It adds a lot of hop flavor and aroma.