How far you can walk in 15 minutes time depends on how fast you're walking.
But for most people who walk at a normal or moderate pace you can walk up to 1 mile in 12 minutes time so in 15 minutes you should cover a little over 1 mile in the 15 minute walk.
I go walking every evening for 30 minutes which means that I walk 15 minutes one way and then 15 minutes coming back so I cover a little over 2 miles in that 30 minute walk.
15 minute walks which covers around a mile are good for your health but if you can try walking 15 minutes one way and then 15 minutes on the way back to cover 2 miles and your health will be much better.
You'll also lose more weight that way if that's what you're aiming for.
The further and longer you walk the better shape and health you'll be in which is important for living a long healthy life.