When they start smoking oil.
No point in doing premptive maintenance unless the shaft sounds like it’s starting to clip against the inside of the turbo.
Rebuilding a turbo isn’t cheap and it’s rare that blown ones destroy engines, more often than not it’s due to a bad rebuild not due to wear.
As for getting a babied turbo car, honestly don’t even pretend they exist.
If someone shelled out the cash for a turbo car they beat on it. If they didn’t plan to beat on it they would have bought the NA version and saved $10,000.
Also don’t ever buy a used turbo. Unless you go all bling BB the regular thrust bearing turbos are like $500 new from the right people.