The magic number is approx. 600-800km, but depends on a few factors including your weight, biomechanics & running style, the terrain your running on & environmental factors.
How well you look after your shoes also influences their life. If your shoes are constantly wet or left in the sun all day they will age quicker then those better 
looked after. There’s no need to tuck them in at night (after all they are just shoes), but keeping them in a cool dry place will keep them at their best for all the km ahead.
While some people use a time gauge, say 6months, this is only useful for consistent runners who can accurately estimate their total km. If you’re running 30km/week in the shoes then 6months may be right for you. Even if you aren’t running a lot of km it’s wise to replace your running shoes after 12months.
Some runners put themselves at risk of injury by not changing their shoes often enough, while others will constantly have 2 pairs going at once. One older pair that is on the way out & a newer pair that is being worn in. By the time the ‘old pair’ are retired the ‘new pair’ have run a few hundred km & become the older pair to the new ones & so the cycle continues.
How long do running shoes last? Do you judge yours by time, km, feel, or their look?
Personally I have a few different shoes. I have my Mizuno road running shoes (reasonably new ones) that I do all my long runs in and most of my med & hard runs. I also have an old pair that are my back up shoes for when it’s torrential rain or muddy conditions. Unless it’s a long run I’ll use the back ups to prolong the life of my good ones. I also have a 
pair of Salomon trail shoes that I use for off-road running and cross training, saving my good Mizuno road shoes just for running.
Light weight runners are designed to be as light as possible while still giving you everything you need for the run. If you use them as ‘everyday shoes’ the breathable light weight fabric will wear away before the rest of the shoe itself so save them for running.
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