I'm running twice a day and getting though loads of running trainers. I run around 10 to 16 miles each day, 6 in the morning and 10 at night, so on average 90 mpw.
The nearly new pair were only at 500 miles, and just started to cause rubs on the side of the foot, so got on of the new Nike Zoom Max out, I bought 3 pairs recently .
Is 800 miles around the maximum i can expect, the 500 miler ones are not worn yet, but just the side causing grief on long runs. I suppose can use these for the gym, as they are clean and use the other pair for xc runs or muddy tracks.