I think the general rule of thumb is no more than 500-600 miles per pair of shoes.
You probably won't notice your sheos getting a bit tired when you wear them, but if you were to go into a shoe shop and try on a brand new pair, you'd probably notice the difference.
If they are visably tired and worn out, they are probably way past their retirement date.
Hope someone who knowws more than I do comes long to help you out soon!!