How much should you run to lose 5 pounds a week?

It's much more about what you eat than it is about how much you exercise. Unless you control your intake it's pretty much impossible to lose weight simply by exercising. For people of average fitness, one excess muffin will mean one hour running.
So any serious attempt at weight loss has to start with controlling your intake, your diet.
Even then 5 pounds/week is a lot to aim for.
1-3 pounds week will be a lot healthier and doable.