I Have Pcos. I M 28 Years Old And Trying

I have pcos. I m 28 years old and trying to conceive. I have been given apcod powder for three months, folic acid and duphaston from day 21. My cycles vary in length from 37 to 45 days. Doctor told me to try to conceive naturally for three months before putting me on clomid. My question is since my cycle are quite long I may not ovulate by day 21. If ovulation happens it will be quite late. How do I conceive if I start taking duphaston from day 21