I think it really depends on the sort of hops you use for dry hopping, and not every cider may be able to handle the same amounts of hopping. I used cascade for about 5 days I think, at that point it seemed like enough as I really didn't want to push it.
But unfortunately since I was still experimenting with my recipe and the bottling and after-carbonating the nice aromas seemed to be lost on account of not bottling it right after hopping and it was also annoying to filter the hops out since I had racked it into a glass jug and I should have just used my bucket.
Also I wasn't quite able to prevent some oxidation of the hops which can give it a slightly 'feety' touch rather than the more fresh, fragrant and ultimately grassy tones.
Anyway cascade seems to be one of the ideal sorts of hops to use for dry hopping, maybe if you need another week yours are less aromatic or you added less?
The cider must be really done and dry as you really do not want any gases to take the aroma with it. If the hops seem to break down after a week, like jimbo said, perhaps this could be slowed down by putting argon in there. No oxygen, apart from the dissolved oxygen that is, should mean much less oxidation.