How many times a day should I apply sunscreen?

I apply sunscreen every morning before going to work. Even on cloudy or rainy days, I apply Dr. G sunscreen every day before heading to work.

Also, on days when I have fieldwork, I apply sunscreen one more time at lunch. Since I am a man, I don't pay much attention to it and just apply it mainly to my face, but lately I have a question. How many times a day should I apply sunscreen? And how much time should I wait between applications? I would appreciate the expert advice of someone who knows about this.