It all depends on how often your grass is growing and how much rain you get as the more rain you get the faster your grass is gonna grow and the more often you'll need to mow your lawn.
On average my lawn needs mowed at least once per week or once every other week depending on how much rain we get.
When I water the lawn to keep it green I need to mow the lawn at least once per week but without watering I only need to mow every 2 weeks.
Usually when I grass grows to 3 to 4 inches tall I then start cutting it but before then it doesn't usually need mowed.