when would you say Montag had the big realization society was wrong and he was unhappy etc. in the book??

Montag realized he was unhappy shortly he and Clarisse met (the same evening).

He was not happy. He was not happy. He said the words to himself. He recognized this as the true state of affairs. He wore his happiness like a mask and the girl had run off across the lawn with the mask and there was no way of going to knock on her door and ask for it back.