How many miles should I do in my trainers? I've had them about 7 months but they still appear to be in good condition and I'm not experiencing any discomfort. I would guess I've done about 300 miles in them.

500 miles tops in any trainers.
How many mpw are you doing?

I agree, about 500 miles. What i do is have one pair for races and 2 pairs for training. Once the racing shoes have done about 100 miles they become training shoes and i get a new pair for racing. This works quite well as long as you don't race more than you train. The old trainers become my everyday shoes.
When you find a shoe that you like wait until the next model comes out and buy several pairs of the old model cheaply. I had 5 pairs of keyano's at about £75 that i am working through for the next couple of years.
Lighter runners may get up to 1000 miles, especially if you run mainly off road, but if you don't replace them soon enough your joints will take too much strain.

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it's not how fast, it's how often

500 miles on the road would really be pushing it I'd say. Even if the trainers look okay, it's the foam type stuff that can be damaged and not provide the support it used to.

I'm sure I've seen a recommendation somewhere of 6 months or 500 miles. I've got a dodgy knee (tear in the meniscus) and I definitely feel it when I move from old shoes to new. However, even after saying that I think I still take most of them to well beyond 500 miles (current shoes are hitting around 750 miles but are being replaced - just the new ones are giving me blistering problems!)

I would disagree a little with the 500 figure, and rather say 400-600miles depending on the shoe, the weight of the runner, the surfaces, gait type. An 18stone plodder doing 13-14mins/mile landing heavily running in a pair of Wave riders is going to get a heck of a lot less than an 8stone whippet running 6min/miles in the same shoe.

Runners Need has a sign saying 'see you in 500 miles' on their counter!
You just can't tell on shoes. Enhanced EVA like Brooks S257 or Asics Speva will last longer than regular EVA but it does depend on the runner. Some very light people are very hard on their shoes! I know one 8.5 stone sub 30 minute 10k guy who hammers a pair of shoes in 200 miles!
I get 3 months out of a pair of trainers usually. I try to rotate between three different pairs. Currently a Brooks Trance 5, Adrenaline 6 and a pair of Axioms. Off road in some nice Brooks Cascadia. Rotating is the way to longevity!