As a lifelong runner it's best to do it by mileage not age. Replace them somewhere between 300 and 600 miles (your weight, leg length, form, surface, etc. all influence this). I will add that over time historically the midsole foam usually degrades so age is a factor too. I'd probably suggest tossing them after 1.5 years or so or at least having a fairly fresh pair to compare them to. The above mostly applies to running but should be relevant for walking too.

Most athletic shoes have a short life like that. The trend tends to wear off. Even if it doesn't, you generally loose grip as the compound ages. Also, the insole and other cushioning stuff tends to break down and loose it's ability to absorb shock, which can lead to joint and back pain.

I run a lot and keep three generations of shoes:

* Used for running when new, usually for a year.

* In their 2nd year, for walking, gym and casual use.

* In their 3rd year, for gardening, house maintenance and stuff like walks in muddy places.

It heavily depends on usage and weight. 10K steps is 1.5 to 2 hours of walking.

I'm quite overweight, and my tennis shoes last me ~6 months in good shape.

If you walk 10km+ per day I'd say 1.5 year is too long to keep them. It also depends on the kind of surfaces you walk on.