I am sure they could last UP TO FOUR HOURS under the right conditions, but not in 90+ weather riding 70 MPH here in Florida.
I have the phase change packs, I never really paid attention to how long they take in ice water, I have two pairs, so I take one pair out of the vest and put them into the cooler and put the cold set in my vest and ride on, by the time I have to stop again they are always frozen solid.
When I first got the vest I tried using all four packs at one time, but the two in the back were right on my kidneys and it was two much, made my back muscles start to spasm, so I now just run with two , one in each front pouch.