How long do you have to let the engine run before turning it off, after a gentle run, and a hard run?

Don't really want to fit a turbo timer.

Thanks

 

After a gentle run? Just let it idle for as long as it takes you to pick up your phone and other sh*t. Any longer than that is going to raise coolant temps, not lower them.

After a hard run, you'll have driven round off boost for a good five minutes to let natural airflow cool things off, after which see above.