When to replace running shoes used for non-impact cardio?

LannyM74

When I do my normal cardio routine of walking/running my rule of thumb has been to always replace by 400 miles because I'm usually terrible at sensing that a running shoe is doing damage to me until it's too late and I have an injury. However, an ankle injury has had me confined to the elliptical for the past month. I've probably logged 100 miles on the elliptical...should I take this into account as wear on my footwear or does the fact that it's non-impact negate the 'miles travelled'?