We have set up a trough full of water by our hay store and we leave the haynets on a rack over the trough to drain off for anything up to 3 hours. Mainly to eliminate dust, but it wont hurt Poppy to lose a few KG
I soak overnight, about 12 hours, for the morning, and about 10 hours for the night haynet. I have a laminitic pony. I do however steam her hay when I think nets will freeze overnight.
My other boy has steamed hay as his waistline is OK, but I want to remove dust and hay spores.