When do you get your period when you're on the birth control pill?

You'll have withdrawal bleeding anytime between the second and sixth day of your placebo week, if your pill has a placebo week built in. If you're on Micronor or other progestin-only pill, or if you're on a combination pill with no placebo pills built in, you may not have regular withdrawal bleeding.
If you are referring to when you should expect your period, it should come anytime within the sugar pills (the week with the different color). Most women get their period when on the third or fourth pill of the 4th week but please be aware that as you just started your pills, you may not get a period at all and that is also common. It takes your system approximately 3 months to adjust the the hormones. If you have been taking your pill every day without fail and at or around the same time, you should be fine.