How long does it take for birth control to leave your system?

Variable time: I am assuming that you mean hormonal forms of birth control. These treatments can have effects on the hypothalamic-pituitary-ovarian axis that persist after removal of the birth control. Some people revert to normal hormone cycling immediately, others can take many months to resume cycling. These glands must be communicating to allow ovulation and pregnancy to occur.

Not too long : The amount of time would vary slightly based on the formulation. However, despite the medication being 'out of their systems', many women find that the return of a birdman menstrual cycle can take on the order of weeks to months.