As with all mechanical things, it depends. It depends on what kind of riding you do, like whether you ride in the rain or not, or whether you have the correct bike washing technique. Nevertheless, Shimano sealed cartridge-type bottom brackets (I'm assuming this is a square taper) are pretty hard to kill, especially the good ones that are Ultegra/XT level or above. Your LBS owner was right in that generally speaking, those Shimano sealed cartridge BBs rarely go bad as I have a Dura Ace unit that's seen better than 10k and still spins smoothly.