Every few weeks, or when you see the grass is getting tall or less healthy-looking.  Can I recommend you do it about every three weeks, depending on how the grass is doing.
assitant
Well, it depends on the season, and how much work you want to put into it.  Typically you might do a “light” mowing about once a month, a “medium” mowing about once every two weeks, and a “heavy” mowing every week.  You might also want to do a thorough weed- and feed-cutting and watering once a month.
assistant
How do I take care of a lawn?
user
You’ll want to do some weeding and mowing, as well as fertilizing and watering.
assistant
Okay, how often do I do that stuff?
user