How much time has passed in Chapters 1 and 2? How do you know this?

I would say approximately 10-12 hours. Lennie and George were going to sleep at the end of the first chapter. It is ten o'clock in the morning at the start of Chapter Two.

At about ten o’clock in the morning the sun threw a bright dust-laden bar through one of the side windows, and in and out of the beam flies shot like rushing stars.