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.
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 believe it takes a week... but for the first month I would continue to use an alternate form like condoms as well.