While the general answer is “once per week,” the reality is that it’s not how often you cut the grass but the length of the grass blade itself. You want the grass to be between 2 ½ and 3 ½ inches, and you should never cut more than 1/3rd the length of the blade at any given time (otherwise you damage the grass).
For most homeowners in most weather conditions, this is usually about once per week, but for some it may be a little more and for others a little less depending on the length of the grass and the climate.