Opk faint 2nd line question...

Allison
This is my first month taking an opk. This is the 3rd day in a row where a very faint 2nd line is visible... But I know you ovulate for only 24ish hours right? So am I not ovulating until the 2nd line is dark? Im so confused! And excited because today the faint line is a tiny bit less faint... So is today the big O? 
Anyone got an answer for me?

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