How long for an opk +ve line?

D • Married to my better half for the last 10 years, expecting baby #1 in Nov 2016
I've been opk testing this month for the first time this month but didn't get anything close to a +ve. 
When I use the strip, the control line appears immediately, as soon as the pee washes past it, but the test line stays forlornly blank. 
For those of you ladies who have had a strong +ve, how long did you have to wait for the test line to "develop"?
Test line appears:

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