Very confused! Please help.... (Possible Tmi)

Louise • 31 Casually TTC babe #3
Hi ladies :) this is my first cycle off the pill and today (cd27) I have my first lot of EWCM (really really stretchy) having been on the pill for 10 years I have never had anything like it before... 
But I am getting only a faint line on the opk... So my question is... How long does the EWCM last? And How long is my window to BD before O?
​Thanks for your info in advance :) xx