Need to know where in Brooklyn you are going. Once you have an address use Google maps, it should tell you how long the transit time. To be safe, you can add an extra 20 mins or so to cushion for train delays, taking a wrong turn exiting the subway station, etc.
I lived in Gravesend in Brooklyn for 2 years, which is close to Coney Island. One night I was on the subway and a mom kept telling her two restless kids that they were almost there. I asked where they were going and the answer was, "Brooklyn." Well, they had been in Brooklyn for about 15 stops already. Brooklyn is huge. How long it's going to take to get there will depend on which area you need to go.