This depends on the method of birth control you have been using. If you have been taking the pill, it's normally recommended that you come off it for at least six months, to give your body time to get back to normal. If you've been using the cap or condoms, then you able to start trying to conceive straight away. Other methods, such as implants, depo injections an IUD's require removal/time for the hormones to get out of your system.
Each woman is different and there is no way anyone can tell how long it will take to conceive.