Question: how long does your period last when your on birth control?
Answer: Your body's response to your pill will depend on the type of pill you take, and your own body's hormones. If you're taking a typical 21/7 monophasic pill (where all active pills have the same amount of hormones—check your pack), bleeding may start on day two or three of your placebo week and last 3-5 days on average.