Most folks recommend dry hopping beer for about 7 days. Cider is generally less dense with flavors, so I'd recommend trying a small amount of cascades, and (carefully) pulling off a sample every day until it seems right.
Make sure you wait until primary fermentation is more or less complete and the cider has mellowed to a point you are satisfied with, as the flavor and aroma notes you're seeking by dry hopping are quite volatile and will slowly leak out of solution.
Keep in mind that ciders can take some time to totally ferment, so use your hydrometer to evaluate completion and be sure to taste the base cider before dry hopping. Ciders are sneaky and delicate!