You weigh 170 pounds and walk 5 miles a day how long will it take you to lose 15 pounds?

Depends on what and how much you're eating

Walking 5 miles ought to take you about 90 minutes.

Not knowing your size, that might use up 600 calories.

If your eating matches your needs today, the walking alone would lose you about one pound/week.

But 600 calories/day isn't much.

it's about two donuts.

So if you're over eating with the calorie content of two donuts above your Daily need you're not gonna lose anything from that amount of Walking.

Any serious attempt at weight loss has to start with controlling your eating.