My vet said soaking overnight is fine. It may help to reduce the sugar content in the hay if the water is changed at least once per soak. I have been soaking hay since March, throughout the summer, and have not had a problem.
I soak hay everyday for my IR mare. The easiest way to do it is to buy two new manure buckets and drill holes in the bottom of one like a big collander. Fill the bucket with hat, set it inside another manure bucket without holes and fill it with water (but not overflowing). Let the hay soak for one hour (per my vet) or longer, then drain by picking up the bucket with the holes. The water will drain into the bottom buckert. Feed the soaked and drained hay to yor horse.
This was the method I used when I was soaking my hay.
Used the "sugar" water on my trees.
The other horses would rush me to try to grab Woodrow's wet hay - they loved it!
I don't soak anymore as Woody gets ODTB cubes and is able to also have some alfalfa.
"And I will be an embarrassment to all
Who have not found the peace in being free
to have a horse as a best friend."
For me, the muck bucket colander method was too heavy and cumbersome. Plus the drilled one would get stuck in the other one, which was frustrating! Since I feed my IR horse in small mesh hay nets, I just weigh out his hay in the net then soak that. Put filled net in big Rubbermaid tub, weigh it down with an old covered bucket with a rock in it, fill tub with water ... Then later I pull out the net, let it drain some, and feed.
I'm thinking about putting a laundry sink in the corner of my wash rack to soak in, so I could just drain it in place (currently have to go dump the tub in the ditch). Long as my flakes would fit in the sink ok, which I need to check.
Just joined this forum and read your post from August, 2012 about soaking hay. Hopefully by now you've got a good system working for you.
If not, perhaps my method will work. Here's what I've standardized on for my Icelandic gelding and two donkeys. I soak 1/2 to 2/3 small bale of local grass hay/day.
I put the hay in my metal round-end trough, which is around 100 gal. capacity. I lay three decking planks on top of the hay, then put a 5 gal. water bucket and a chunk of concrete on top of the planks for weight. I fill the bucket after I put it on the planks so I don't need to lift it full and I use that water in the stall bucket when I'm done soaking. I fill the trough nearly full and let the hay soak for a minimum of an hour up to 12 hours. When I'm done soaking I drain the trough but keep the hay inside and pull out what I need for feedings. I tilt and prop up the trough so it will keep draining. Sometimes I set the hay I want to feed on something to drip, but usually I just take it straight to my feeder tubs and nets. I have a good place for my trough where I can easily drain it down a slope and access my hose. Our climate is rainy in winter, so ice isn't an issue. Come hot weather I might have to soak smaller amounts to prevent spoilage, will see what happens.
At first I tried soaking hay in smaller buckets, nets, etc. but those methods didn't work because I had to lift heavy wet hay. The way I do it now I can pull out soaked hay in smaller amounts that are not heavy. It took my animals a day or two to accept eating wet hay, but now they love it. I'm soaking to reduce sugars from the 15% starch of my tested dry hay, hopefully to 10% or less.