I believe it takes a week... but for the first month I would continue to use an alternate form like condoms as well.
i know with mirena my ob said it takes 48 hours for the hormones to work through the body but told me and dh to continue using condoms for at least 3-4 weeks just to be safe.
I'm under the impression that for the first month you should continue to use an alternate method of bc...I do think it gets into your system within a week but most obs/drs recommend the better safe than sorry method for the first month.