For me, it depends on the beer. For a less "hoppy" beer like an American Pale Ale, I usually use 1 oz for 5 gallons. For IPAs, 2-3 oz for 5 gallons. In an IIPA, I have used 5+ ounces.
It all depends how much aroma and flavor you want to add. The variety also plays a part. I've found that some varieties like Chinook or Amarillo have stronger tastes when used for dry hopping than a Centennial or Cascade.