It will depend on the salt manufacturer. One of them (I think Instant Ocean) makes it kind of simple, and just instructs to add 1/2 cup per 1 gallon of water, so for a 5 gallon bucket, that would be 2.5cups of salt mix.
Other manufacturers like RedSea, dont give a cup amount, instead they instruct something like 1.4lbs per xx gallons (dont have the bag handy as Im at work), and still others just say mix in enough till you get a SG level of 1.021 to 1.024
Personally Ive now tried three different salts in the past 8 months, Instant Ocean, RedSea and now some other salt that my LFS recommended as being the quickest to disolve. And so far, I've used the simple method on each, and used basically 2.5cups per 5 gallons that I make up, and my SG is usually 1.021 to 1.022 for each batch.
So in short, it seems that the 1/2 cup rule seems to work, at least it has for the three salts that I've tried.