I probably have a few pair that I rotate for the tm, trail, and asphalt surfaces. I notice trail running (due to small packed rocks) will be the first ones to go because I do majority of my running in them. I really think it depends on your surface and time of year. In the cool months, I don't seem to go through them as much as I do in the summer months.
Like others, I buy a new pair when I lose the cushion or if I start to get more aches/pains. I'm just glad to hear that I am not the only one who goes through a pair quickly.