It honestly just depends. Assuming that I follow a very ordinary, routine week with no deviation in activity (that might require me to drive someplace I don't often go to), probably every 9-10 days. In a typical week, I do a lot of city driving as I run errands and frequent my usual places around town. Three days a week, I take a commuter bus to work so I only have to drive 5 minutes to/from the local park and ride. The other two days, I have to drive 20 miles to work so that's where I pick up my highway mileage.
My friends are all scattered about in different areas. My family is a good 20 miles away. Any doctor's appointment, hair cut, or other miscellaneous events can also add in a decent amount of mileage since most of that isn't close by. So if I have an unusually busy week I might find myself driving all over the place, in which case a tank of gas might barely last me one week.