How long does it take for Plan B to be absorbed?

What do you mean do I know when it doesn’t work at all?

Will explain properly when I'm home, but if you're already ovulating you need one of the other methods of emergency contraceptive; either the other emergency pill called Ulipristal Acetate or to have an IUD fitted by a doctor. Either of those things can be effective if done within 5 days/120 hours of the unprotected sex.

I had started my period a little bit that night and then fully got my period the morning before I took Plan B One Step. My cycle is normally 29 or 30 days so plan b should have worked correct?

Plan B wasn't necessary so didn't need to work. You are not fertile when on your period. Not if you get regular periods. At the start of your period, you wouldn't even need Plan B to cover you to delay your ovulation, as you were too far away from it. It will have done nothing useful, but may cause you a lot of hormonal side effects & irregular periods. It's a huge dose of hormone for your body to deal with.
It's worth learning how the menstrual cycle works, as it could save you hassle & stress in future.

How will a woman know when they are ovulating and how do we figure out how many days in our cycle?

Her cycle runs from the first day of her period, until the day before her next period. If her cycles are regular.
Ovulation can be worked out by charting fertility symptoms each day for a few months. You can learn online or at a clinic how to do that.
With a regular monthly cycle (28day) ovulation tends to happen around day 14.