Hi Wobbly
I'm not sure if I understand you correctly.
Are you under the impression that as the disc loses its height and the two vertebral bodies move closer together that they will fuse together because of that?
In fact the opposite could happen and there could be more of an instability because there could be an increased laxity in the little ligaments that are usually taut when the vertebrae are not as close together. I hope this makes sence?
Anyway, the closer the bony parts are to each other, the more friction between them and this causes osteo-arthritic changes, which then will cause the joints to stiffen. But generally the joints don't physically fuse together.
And no there is no way of telling timewise what the progression of this would be.
I hope this answers your question?
Greetings