On replacement, since you said you replace when they feel dead - that is a better method than the mileage numbers everyone quotes, and worth defending.
The commonly repeated range is very wide in practice. It depends on your weight, your gait, the surface, and enormously on the shoe. Some lightweight racing foams are done in a fraction of the mileage of a firm daily trainer. A single number applied to all shoes is not useful.
Better signals:
How it feels on a longer run, not on the first kilometre. Dead cushioning shows up as unusual leg soreness after distance before it shows up as anything you can see.
Compression lines in the midsole, and whether the foam still springs back when you press it hard.
Wear pattern through to the midsole at the outsole.
Upper failures, often what actually ends a shoe rather than the foam.
If you do want a number, log the mileage per pair so you learn your own figure for the shoes you actually buy. After two or three pairs of the same model you will know it better than any general advice, and rotation makes that logging slightly more work and considerably more useful.