How long and how often can a woman look at pictures of naked women before she realizes she's gay?

Your question is "Before SHE realizes she is gay." You may think she's a right now. But to answer your question: it could be 30 years, 50 years, or never. IF her religion has taught her that liking other women is wrong, or if she knows this would make her mother stick her head in the oven or give her dad a heart attack, if she really loves her husband, at least like a brother or whatever, and if she cares about her social standing in her small town.... all these things will keep blinders on her eyes and make her live whatever cookie-cutter life she has forced herself into. As your question is actually a sociological one, not a psychological one.