Before meals and after using the bathroom are usual times to sanitize your hands. Another great place to sanitize is after exiting super public areas, like the subway here in New York City.
Be aware that there are some disadvantages to over-washing or over-sanitizing hands, most notably chapping and dryness. It is important to be aware and listen to your skin. If you find your skin is overly dry, then you're definitely washing or sanitizing too much.