I used to believe hay should be soaked for 20 mins, so tried to do this for years- although very difficult in the winter when time is short, as it would take another 15 mins for it to 'bleed' the water off before going in the stable.
Then last year CW developed a mild form of COPD after the hay was put in the barn behind him.. vet came, gave me medicines, advised on how to deal with it re keeping the stable clear of dust, spores etc. I asked him how long I should soak hay in case it needed to be longer than 20 mins and he said 5 mins soaking will be sufficient. .
Have kept to this (unless I forget it and go hacking.. oops) and I haven't seen any change in his breathing/coughing. in fact he doesn't like the 20 mins soaked hay now... well not as much anyways... (he's a slave to his stomach!)