An important part of being any kind of runner, but especially a distance runner, is tracking your mileage.
Think of it like your car: you track your mileage because it’ll (in theory) give you an idea of when you need to fill up the tank. Likewise, you need to track the mileage on your shoes.
In general, running shoes have a “shelf-life” of 300-500 miles (wide range, no?). Various factors like the surface you run on & the amount of miles you do per week will affect how quickly you need to replace those shoes. Now, depending on the kind of shoes (there are support, neutral, motion control, etc), you might get more miles. For example, I usually get more miles out of my support shoes than out of my neutral shoes.
That said, you’ll usually have an idea as shoes start to wear. The in-soles will have the indents from your feet. If you let them go too long without replacement, you can start having issues. I had an old pair of shoes actually take me off my feet for a full 24 hours (thankfully it was over a weekend) due to painful, swollen ankles.
Now, I not only track the number of miles I put on each pair of shoes, but I keep a close eye on what the wear looks like & if I start getting any consistent pains in them.
My current support shoes, Asics Gel Kayano 17s, are at about 6 months & 400 miles. I last double checked the mileage around 350 so it’s about that time. Ironically, they have started to cause pains in my joints (see my next post “Keep an Exercise Log” for my explanation of why I know the shoes are the cause). So I can stop wearing those & buy a replacement pair now, instead of later when I’m injured.
This Running Shoe FAQ over at Runner’s World has a nice long list of good questions with good answers. The first page can probably answer most concerns over when to replace shoes.