Hollowtech II bearing life can vary greatly due to installation errors. That plastic threaded plug on the non-drive side is for bearing pre-load(really just taking play out of the system) and too much pre-load can destroy the bearings in a very short time. It should only be finger tightened until there is no side play in the crankset. You should get way more than 3500 miles out of a properly installed Hollowtech II.