When should testing with opk begin when ttc with pcos?

I was told to start testing around day 12 also when i was first diagnosed with PCO 10 years ago. I wasn't on metaformin or anything like that though at the time as they were just discovering how to treat it. I started clomid once diagnosed though but never ovulated on it. But yes I was told to start testing on day 12 with the OPK.