Plan B One-Step - How soon before ovulation is plan b effective? How long am I protected by Plan b?

It will delay ovulation for how long isn't really known. Stop over thinking it. As long as you don't ovulate before you take plan b it has a chance. If you've ovulated when you take it, then you can hope for the effect of thinning the lining. No you should not take another dose.

Plan B claims that it may delay ovulation, & also that it may prevent implantation of an embryo, by making the lining of your womb unsuitable for a fertilised egg, so you should be protected by Plan B wherever you are in your cycle. This is especially so, because you took it so quickly after unprotected sex.
Taking another dose of Plan B will just mess your body up for weeks. You can't take it twice in the same cycle, for this reason. It's a BIG dose of hormone.
From the 1 dose of Plan B you have taken, you may experience bleeding in the next few days, & your period could be late by up to a week.
The bleeding you get on both occasions could be very heavy. Occasionally it can be lighter.