Opk question

Myranda
This is my first time using opks...been ttc for a year. Ive been testing since day 10. I normally have a 35 to 40 day cycle....ish i was testing twice a day during what glow said was my peak frame . I never got a second line as dark. How long does the peak lh surge last? Is it possible i somehow missed it testing 12 to 14 hours apart?