If it is just for dust 10 to 20 minutes is enough. That is how long I soak my old lads for and he is fine. In sayting that the hay I have is not very dusty anyway.
Thank You problem is the hay is real good and he loves it so after I soaked for 10+ hrs he turned his nose up at it. I will do for 20mins and I feel too that this is long enough. Thanks Again.