How often will depend on what your soil needs as well as what other management strategies you use, and the only way to know that is to have a good, reliable soil test done. Manage the nutrients in your soil, and the Soil Food Webs ability to deliver those nutrients and that wil take care of itself.
Sufficient amounts of organic matter in the soil, mowing high to leave as much leaf exposed to the sun, so that leaf can manufacture nutrients to feed the roots, and watering properly will go a long way toward a good, healthy turfgrass.

Without a soil test, which I don't necessarily suggest, then "normally" a cool season lawn should need fertilizer in late spring (now-ish), Labor Day, and Thanksgiving. I would try that schedule this year and see how it looks.