In short 1 hour of soaking loose hay in twice the amount of water at room temperature = a reduction of around 31% of 'sugar' in hay.
Katy from Safergrass states 2-3 hours in 'lots of water'... try to aim for twice as much water as hay to leach out as much WSC as you can. After a few hours the WSC in the water will become the same as the amount in the hay so no more will be leached out, then you are looking at loosing natural nutrients and maybe fermenting the hay, which in turn could cause a colic risk. If the horse has foundered I read somewhere that you can then remove the water and re-fill for another 20mins to rinse and take off any surface WSCs that may remain. I soak overnight as it is convienent for me and as the temperature is colder the process takes longer, when the weather warms up I will convert back to soaking for a few hours as the water warms up quicker!