Question about cm

Nicola • Ttc #3 since march 2014.
I just started checking my cm this cycle and noticed today it changed from a white lotion like cm to clear slippery ( I am assuming ewcm).
​I was expecting to ovulated on cd 17/18 but now on cd 21 and still no +opk.
​So my question is, from the first appearance of ewcm, how long can I expect to wait on a +opk?