I look at it this way...
5 - 15 lbs gained will be water depending on the gear and anti-e's used
The rest is based on your diet. If you eat 500 calories above maintainance you should gain nearly 1 lb per week. If you eat 1000 calories above maintainance, you'll gain nearly 2 lbs per week (the reverse is true for cutting).
The type of food and ratio of macro nutrients (pro/carb/fat) and workout will determine the percent of muscle to fat ratio you will bulk/gain or cut/loose.
I know it's pretty simplistic, but that's my approach.