I hooked #8 steel hook into a stick just below the water about 3 months ago, and it's still there. I think it would dissolve a little quicker in a fish, but still take a month or more to drop off. I bet big hooks or nickel plated hooks take a lot longer to rust away.
Here's a couple of pictures of a #10 bonze steel hook that made the "trip" unscathed.....
Don't know how long it took however, but I could have tied the hook back back on and recycled..
Another good reaason to mash the barbs...