Really, don't even use it. It's a waste of money, and as long as you give your car some TLC before you turn it off, it's not even needed.
Like I said, Sean Morris, owner of RBmotoring, he does NOTHING but tune Skyline GTRs day in day out, and he doesn't use one, and he even says use your "money on something better, like strippers"
I haven't talked to Nick Wong directly about this issue (he was in this year's SCC USCC with his R32 GTR) but his car was tuned personally by RBmotoring, so I would guess that he doesn't use a turbo timer either
I personally won't use one when I get my car with the CA18DET. But I have found a better solution to a Turbotimer, the Compustar system, has a system where the car will run until you open and close the door.
That should be more than enough time for modern BB turbos to "cool down"