Once you stop birth control, some are able to get pregnant right away but some take as long as year (depending on what you were on).
Basically your flushing out the hormones from your system. If after a year, you are not pregnant, then consult a doctor. Assuming of course you have access to either sex or sperm.
If you used an IUD though, you may want to check with your doctor earlier. Some IUD's can mask hidden infections that can reduce fertility. This is not common but is a concern. All IUD's must be removed for a safe pregnancy.
If you wish to improve your odds of conceiving faster, you can pick up an ovulation kit at most pharmacies. It's will tell you when you are ovulating and have the best chance of getting pregnant.