Hey Mommev38!
I had my fusion on Dec. 29th, and when I went in to my post-op appointment on Jan. 29th (one month later), the doctor showed me in my X-Ray where I had already begun to fuse. I was not NEARLY completely fused at that point, just barely starting, but it was there.
Very fast, but it's probably becasue I was 19 at the time. I've heard of people who take 4, 5 months until their fusion starts to show. Best thing is not to stress about it as it will just put extra strain on your body.
Wish you the best in future appointments! Take care!