Around how long does it take a bone spur to grow on a facet joint after an injury?

May never grow: A bone spur is generally a result of chronic, long term wear and tear, not necessarily an acute injury.

No rules for that: If you have a bone spur it may or may not grow. Unfortunately you have had bone cancer and that causes a completely different concern for any unnaturel growth. It also showed you already have chronic pain and it may have nothing to do with a bone spur. Many people have bone spurs that show up on mris and ct scans and they are totally incidental and do not cause pain.