I'm running a mid quality sram 8 speed chain on 9 speed XT rings and a 8 speed XT cassette. My MTB, which never sees pavement, has over 1100 miles on it's current drivetrain without excessive wear.
The conditions you ride in, the frequency of cleaning & lubing your drivetrain and your choice of chain lube are probably the most critical factors in drivetrain longevity. I normally perform weekly preventative maintainance on my bike, but in some trail conditions I clean and lube my drivetrain after every ride.
2003 Iceman Challenge - 2:34:55 - 897 / 2,000*
2002 Iceman Challenge - 2:39:23 - 1093 / 2,186
2000 Iceman Challenge - 2:49:18 - 1516 / 2,153