It depends on what you are refering to...
My 108 with the old M130.920 lasted 260K+ with the valves only being done once. It was starting to blow some more smoke when I let her go, but I am sure with another valve job she would have kept going. That was with MB technolgy from 1970. I would guess a properly maintained modern MB engine's bottom end would last over 300K. Maybe a search through one of those "who has the most miles" posts would lend some insight...
1996 (W210) E300D 86K - Traded in for a Lexus
1992 (W124) 300D 2.5 Turbo 202K - Sold
1983 (W123) 300D, 146K - Sold
1970 280S, 263K - Sold - Beginning of addiction