It depends on the weather, the zone you are living and season. In spring we mow in every two weeks. From July on wards in every six weeks. Luckily we don't need to mow in winter as we don't have a lot of rain during winter.
Here in Northern VA where I am, I have to mow at least once a week. This summer is particularly important. The grass grows so fast! With the very, very mild winter - we have more bugs outside then normal - spiders, ants and earwigs. We have to keep the grass short so we can put down the outside bug killer. It's going to be a LONG summer. :-)