When do you start taking opks on your cycle?

Heather

I have recently been diagnosed with PCOS and my last cycle was 50 days long so it's very unpredictable so want to take opks to hopefully have a better idea of things but I can't track temperature due to being up in the night so much 😩 what day do you usually start taking OPKs? Straight after period is done? Or around cycle day 10?