when to start taking OPKs

Hi! So today is the last day of my period (day 5 of my cycle). My husband and I have been TTC for a few months now. This month we wanted to try OPKs for the first time. My cycle is anywhere between 30-35 days. How early do you start taking the OPKs? I know on a typical 28 day cycle, day 14 is typically when you ovulate. But with my cycles anywhere between 30-35 days, when would be the best day for me to start doing the OPKs? Thanks in advance!