Depends on your age. If you are under 35, you could start immediately. If you are 35 or older, some might recommend former-smokers use progestin-only birth control (pills, implant, IUD, shot) until you quit smoking for an entire year. In general, it takes about 12 months for heart risks associated with smoking to be cut 50% after you quit smoking. However, you could probably start estrogen-containing birth control sooner depending on age and how much you smoked, just know there are increased risks of blood clots and discuss/mention this to your doctor.