The effectiveness of birth control depends on the type and the timing of when you start taking it:

• Combination birth control pills (which contain estrogen and progestin) are effective immediately if started within the first 5 days of your period. If you start at any other time in your cycle, the pill takes 7 days to become effective, during which you should use a backup method like condoms.

• Progestin-only pills (mini-pills) are effective immediately if started within the first 5 days of your period. If started later, they take 2 days to become effective and require backup contraception during that time.

• Other methods vary: copper IUDs work immediately upon insertion, hormonal IUDs and implants take 7 days to become fully effective unless inserted during the first 5 days of the cycle, in which case they work immediately.

• Barrier methods like condoms or spermicides work immediately when used correctly.

In summary, most hormonal birth control methods require up to 7 days before they are fully effective unless started during the first 5 days of menstruation, when they work immediately. During the waiting period, it is recommended to use a backup contraception method. This timing assumes perfect use; missing pills or starting mid-cycle without backup reduces effectiveness significantly.