The rule of thumb is that the optimal height for grass in the cool season is about 2 ½ inches. With each mow, you should cut around a ⅓ of the top of the blades of grass. It’s best to cut your grass when it reaches about 3 ⅔ inches in height.
In the summer, it’s best to keep the lawn a little longer if you’re unsure of the height. If it’s very hot, use a higher setting on your mower and keep it at that height until the fall.