Different products work for different people. Acne's cause is different for each person. Your dermatologist will analyze your skin and prescribe what is appropriate. You will need to use it exactly as they tell you to. It may take a couple of weeks before you see results though so do not be disappointed if it does not work over night. Be patient and use what they prescribe in the manner in which they prescribe it. :0)
ScoobyDoo, you seem to be focusing on how long it will take. What's important here is that you use the products as prescribed on a regular basis. Not being consistent will make the product ineffective. Also, going to a Dermatologist is a process. They will prescribe a treatment and then see you again in a few weeks. If it's not working, they will prescribe something else. Every patient is different, so no one can answer 'will it work' and 'how long will it take'. Only you and your Dermatologist can work together to get to the end result you desire, but it will not happen overnight.