What should I expect from Plan B if I don't know when I ovulate ?

In that situation, I would have used Ella (Ulipristal Acetate), not Plan B. Ella is the only pill proven can prevent pregnancy during the 24 hours around ovulation, itself (you can see that on the Manufacturer's website). Plan B might, but it has not been proven. As you don't know when you ovulate, Ella would probably have been the safer option, if available to you.

The only way Plan B is proven to work, is by delaying your ovulation, & that isn't guaranteed. It may work in other ways, but they have not been proven.

However, now that you've taken Plan B, if you're wanting an extra precaution, in case Plan B fails, you could get a Paragard IUD fitted by a doctor within 5 days/120 hours of the unprotected sex. That can prevent a pregnancy from implanting, & be left in place as longer term emergency contraception.

IUDs can be painful, at first, for some women, especially if they've never given birth.