How long can plan b delay a period if taken a few days before expected period?

1-2 weeks: Plan b is a large dose of progesterone. Depending upon when it is taken during the cycle, it certainly is associated with not only breakthrough bleeding, but also delays in your normal cycle. If you take it right before your period, my guess is that your period will be delayed up to 1-2 weeks. You should also check a pregnancy test in 2 weeks.

Not at all: Plan b by itself will have no effect on when your period comes. If you are seeing a delay then you need to do a pregnancy test.