About once a week. We're more concerned about the impact on our waist lines than our finances.
One thing we try to do to save money is to order meals for leftovers. As an example, a $14 burger will almost always be one meal, while a $14 Thai curry typically lasts for 3 meals or more. Even within specific restaurants, bowls or plates offer more bang for your buck than sandwich entrees.
We also often roast vegetables or other side dishes in the oven while waiting for delivery, mainly for health but it's also typically cheaper.