All things being equil a turbo will have about 1/3 the life span the diesel engine does.
But nothing is never equal in life is it? Bad fuel and improper fuel delivery upkeep causes more diesel failure's then turbo's. In over the road trucking, turbo's are lasting about as long as engines now thanks to modern oils and proper maintance.
It is common to see the A4 Jetta, Golf, and New Beetle TDI diesels hitting 200,000 miles with no turbo problems as long as the Variable Vane turbo is excerised regulary to keep the little acuating rod from rusting were it will not open fully. The rusted rod however causes no turbo failure and can easily be fixed. What has cause a lot of failures is some improper mfg turbo's. The seals were either bad or improper installed, or died early because of non synthetic oil use.
On the VW TDI's they seldom ever have EGT over 900* F, unless they have been modified or are being run real hard in the mountains or pulling heavy trailer's, then you see EGT in 1100* to 1150* range. But for short periods that's not a problem as long as a FULL senthetic diesel rated oil (soot control) is used. The same should be true for the 123 turbo's even though they have a wastegate system (VW went back to wastegate on the 04 models) and if you want to really reduce the EGT on the car then reduce the back pressure on the exhust system.