How long would it take for me to get a green card if my mother is a green card holder?

It can take many years. You should look for other ways to come sooner. For example, Pakistanis are eligible for non-immigrant investor visas, etc.

As a legal resident, your mother's petition for you will take about 7 years to become current. If you marry while this petition is pending, it automatically becomes void as there is no visa category for married sons or daughters of green card holders. But if you marry a U.S. citizen, who files a petition for you, your wife's immediate relative visa petition can replace the one filed by your mother. A visa becomes immediately available for you and if you are physically in the U.S., you can concurrently file for green card at that time. In addition, you might also be qualified for either an immigrant or non-immigrant visa in the employment category as a physician. Please consult with an experienced immigration attorney to advise you of all of the options that may be availing to you.