i have a volvo XC90 T6 twin turbo. How long do I need to keep the engine running to slow down the spinning turbo before I shut off the engine? I've heard that shutting off the engine immediately after a drive may ruin the turbo. Is this true?

I believe thats what the electrical motor is for when you shut off the car but I'm not certain. I do know that many of the new turbos don't require a cool down on the rotation. I drive a fire truck for a living and I DO have to wait to shut it down after driving but its an old turbo.
:)

That advice to idle while the turbo cools down is left over from the past and before water cooled turbos. There may be an exception if you've been pushing the engine for a while (long uphill or high speed run) in which case you could idle for a minute before shut-off. Normally the water circulating by thermal siphon will cool the bearing after shutdown and even a short drive at lower speeds before shut-off takes care of it.