Whenever I dry hop I do it for the whole time the brew is in seconday.
If you want you could put them in for the last week or so. I've heard some people complain about off flavors if you leave them in for more than a week, but I've never had a problem leaving them in for up to 3 weeks.
I usually drop the hops in about a week before bottling/kegging. Then, I taste it. If it's enough, I go ahead. If not, I add more hops. I've never dryhopped with the same hops for more than about two weeks, though.