Some birth controls take 1 week to kick in, some 2, some can even take a whole month!
So it depends on the pill, the nurse should know what she is talking about though, so 7 days, then its safe!
Read the information sheet that came with your birth control, it tells you everything you need to know about your pill!
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My doctor told me to wait a month just to be safe.

ALL combined birth control pills only take 7 full days to put your ovaries to sleep.

You can read this info on BC pill websites, and in the actual leaflet that comes with the BC pills themselves.