I got almost a year and roughly 40000 hard km on em, and there is still all kinds of tread left on my bfg kms. No cupping, scalloping or otherwise odd wear.
I figure at least another 40000km, probably closer to 60000. So 80-100000km I figure.
(100000km = 62137.119mi)
Others may have better numbers but I'm a hick so I likes to spin em up. Plus they spend probably 25% of the time in the mud/dirt/snow/rocks. Thoroughly used!