Just go on feel and fit?

mileage is down to weight, biomechanics, yerrain, pace of the runner so no magic number

Some soles shrink slowly over time as the rubber dries making it less bouncy. The shoe can become a size smaller over time which can get risky for toenails

Depends on the shoe. Here is how I manage my shoes.

Wear them down on concrete/tarmac. Then...
Wear them down on treadmill/soft track. Then...
Wear them down on grass/sand/ cross country running.

This extends the life span of my Hokas and they feel about the same through out. Helps with shoe rotation too.

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Indoor Triathlete - I thought I was right, until I realized I was wrong.

If you are consistent in your running is is very easy to figure out. If you one day go out for a run and that evening or the next morning you are really sore, check the boxes. Normal run, no big changes in pace or geography ( you didn’t bomb down a big hill that you don’t normally run). If you can check those boxes, it’s the shoes. Don’t hesitate, don’t wait for anything, stop running in them.

The other posts were also spot on. Everyone is different and there is no possible way to give a time or distance on a pair of shoes. The surfaces you run on also play a large roll. Run on the sidewalks all the time and you should expect to be replacing shoes often.

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Dave Jewell
Free Run Speed

Just storing them for a period of time your shoes will have some changes. Storing them in a hot car is really not a good idea. The rubber changes over time and exposure to elements. There is no easy and accurate way to measure the changes in your shoes. When I used to run a lot I would monitor soreness and injuries on a daily basis. There seemed to be a correlation between worse out shoes and soreness and or knee pain.
When you are running 50ish or more miles a week it really doesn't take all that long to wear out a pair of shoes. I am sure that compounds have improved from the 70s to early 80s but I always found my shoes best effectiveness died somewhere between 300 to 500 miles on hard surfaces. So at 50-60 miles a week not long at all. But then when I reduced running mileage with triathlon training down to 30ish miles a week my shoes lasted month wise a lot longer.
I know some of my friends wore their shoes a lot longer than I did. Some got away with it others got injured. I used to write a date with a sharpie when I started running on a pair of new shoes. Because I alternated with several pair of shoes that were just alike I also put a # on each pair. That way I never mixed up a semi new pair with a semi older pair.
My progression was Class A run shoe, trail shoe, wear around the town shoe and then yard shoe. It has been a long time since I have had an army of old running shoes around the house, but I still remember a closet of close to thirty pairs of very similar shoes heaped inside. Bad idea to try and extend the life of your running shoes at the expense of possible injury.

Mileage.

Assuming you are storing them inside (i.e. no crazy UV exposure or sauna-eque temp extremes) they'll be fine). I've had racing flats that lasted me 7years (at ~6 races/year with an average distance of 10k, it takes a while to wear them out).

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

I kept my shoes in the garage in Texas heat and they were falling apart. I store them inside now. You just wear them, run and see. It's all about how you feel. I'm cheap so I wear them until soles wear out or there's a hole on the side near my pinky toe.

Oh I forgot to mention shelf life. Shoes do go bad doing nothing. I found a running store in Charlotte a bunch of years ago when I was there on a conference. They had shoes from at least 10 years ago models, I scooped up a couple of pairs from the wayback machine that I loved when they were new. They were both rock hard. the midsoles were total crap with no cushion at all. I have seen older Nikes blow the air pillows after a long shelf life too. So yes shoes do have a shelf life on a shelf. Not too sure what it is but they do change in the box.

I still have some race shoes with little mileage that I started running on in 2014.

I do 30 to 50 miles a week.
I ditch mine after 300 miles, so they are usually done after 6 months.
Check out the manufacturer recommendations.
Some say 6 months... which is shocking for how much they charge.

I have a home pair for outside, pair by the treadmill and a pair in my travel gear. I have found relatively affordable styles that work and stock up 3 at a time to rotate the 300 around.

I have found past a year old, the cusion starts fading and over 300 miles, they are shot and I start developing foot problems.

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Ryan
http://www.SetThePaceTriathlon.com
http://www.TriathlonTrainingDaddy.com
I got plans - https://www.trainingpeaks.com/...dotcom#trainingplans