When I want maximum whirlpool impact, I'll for 90 minutes. I find that I get more hop character from the longer whirlpool. FWIW, I don't have a recirculation pump, so my "whirlpool" is really just a hop stand with intermittent stirring (every 5-10 minutes). YMMV, based on your system.
I have also started playing around with doing my whirlpools at 120F. I haven't decided whether I prefer this temp over a hotter whirlpool yet, but I certainly like the character I've gotten so far.