Is the turbo leaking oil into the exhaust? You should replace both turbo's at the same time. You need to replace the whole turbo as the bearing fails causing the seal to fail. 20 hours is reasonable to replace both turbos.
The oil is dripping to the hot exhausts that is burning.
Usually how long does the new turbo last - in average? Obviously it would depend on how your drive and care (synthetic oil and frequency of oil change, etc.).
I just bought this vehicle last week not realizing the leak. Too hefty price I have to pay to fix.
Oil is dripping on to the turbo? If it is the passenger side then your cam adjuster seal is leaking or your rear camshaft cap is leaking. If it is the drivers side then your rear camshaft cap is leaking. It could also be your valve cover gaskets leaking. If this answer helped you at all please accept it. If you need more info I am more than wiling to help.