As mentioned before, there isn't one rule of thumb that applies universally. That being said, about 3 ounces would be a reasonable ballpark for a 1-gallon batch. Also, you are better off sticking to grains that don't require mashing to convert their starches to sugar. Caramel/crystal malts will work, as will roasted grains like chocolate malt, black patent, roast barley, etc.
For example something like this would be a decent starting point for 1 gallon of Pale Ale:
1.25 lb Extra Light DME or Pilsner DME
3 oz Caramel 60L (steeped)