Not sure about a fish. I have buried expired chickens into my garden beds, probably about a foot down, and they are fully "incorporated" in less than six months. That is in a rich organic soil, probably lots of microbes at work. I put a rock on the burial spot so I don't accidentally dig them up and to discourage scavengers. I had a bunny who died of old age a few years back and I buried him in my regular clay soil. I found very little trace of him (just some fur tufts) after just a few months, when I decided to plant a tree on the spot, so not sure if the organic matter has much of an impact. With a full body, however, I did find that the soil was wetter in that area from the decomposing juices.
I imagine if you are just burying scraps, they would be incorporated within a month or two, but larger bones (i.e, salmon vs. trout) will take a while.