If the turbo is looked after ie allow it to cool off after a hard run and treat gently when warming up these days they should last as long as the engine, I know I always let my tdi run for a few minutes to
(a) cool down the turbo slowly and
(b) wind it down,
I have heard that the old fashioned quick blip on the throttle before you turn off is a sure fire way to kill a turbo quickly, spins it up then stops the oil / water cooling just when it needs it most!