When do I start using OPK?

Destynee • PCOS • 2 natural born • pregnant with our letrozole baby🤍
I'm on period day 3 and wondering when is the best time to start testing for ovulation. My cycles are somewhat irregular lasting somewhere in the upper 30's for days. Last cycle I got my first peak reading with clear blue ovulation kit on CD20 so when do y'all suggest I start testing for ovulation this cycle?