I agree with Captain Damage. I normally do 5 days and if you want more hop aroma you need to increase the quantity not the time, if you increase time you likely to get more unwanted flavours.
Also dry hopping temp. has an effect on how quickly the oils move to solution. If you was largering when you add the hops the hop aroma transfer to the beer will be much slower because the temp. is cooler . Normally I dry hop my ales at 21C.
Also some people suggest if you want a really big hop aroma to dry hop multiple times, say 1oz for 5 days, remove, 1oz for 5 days, remove, 1 oz for 5 days. This is more for IIPA thoughs not most normal ales. I don't know why this makes much of a difference but apparently it does.