IIRC, Ray Daniels ran a test years ago and the best results were at 185F for 80 minutes. I typically start at 195, then set my Foundry for 175F and let it hover there.
If I'm going to do an extended whirlpool, then I don't bother with less than 45-60 minutes. Any shorter than that, then my results aren't much different than doing a flameout addition and chill per usual.