It is the same as dry hopping in beer. So in primary fermentation for ~week. If you let it for too long it gets grass tones (like bad grass taste).

This one had too much hops (~10g/l) for next time I will add 1-5g/l. As for what variety to choose go for some aromatic hops for dry hopping (it is usually specified in shops/e-shops), I used Summit.