How long does it take to run enough to lose fifteen pounds?

Online calculator. You need to put in your info and use this calculator http://www.Healthstatus.Com/calculate/cbc you can also check http://www.Exrx.Net.
Mix it up. Eat healthfully & be physically active. Aim for 7.5 to 8 hrs of sleep / night. Hydrate w at least 64 oz of water / day. Calories burned must > calories consumed. Do both cardio & weight training. You can mix & match different kinds of exercise. This site helps determine calories burned for > 100 activities: http://www.prohealth.com/weightloss/tools/exercise/calculator1_2.cfm.
6 weeks. A good goal for weight loss is 2 pounds a week. While running is a good strategy, watching your diet as well will help.