Not too bad.

1000 steps takes about 10 minutes.

15K steps takes about 150 minutes.

150 min by 52 weeks in a year is 130 hours a year.

130 hours a year from say age 30 to age 80 is 6500 hours walking.

And so that gains you 3 years, which is 26280 hours, or 17520 awake hours.

So you are gaining about 17520-6500=11020 hours of life to use for things other than walking which is more like just under 2 years in days counting only awake hours.

Now of course it's certainly possible to do things you enjoy while you walk or perhaps things you would have done otherwise while not walking.

For instance I suppose a treadmill desk and using it to walk when you would have just been sitting instead.