Depends on your starting weight. If you are overweight your Doctor will hope for a smaller gain than if you are average..also if you're under weight they'll want more. So could be as little as 15 or as much as 30+.
It definitely depends what you weigh, I found this weight gain estimator online. Maybe you should try this to see what your healthy weight gain will be. http://www.babycenter.ca/tools/weightgain/