Gary, you have already received great advice…do not season it first. More advice is since you want to do a bone in you are running the risk of easily puncturing the bag from an edge of the bone. There are a couple ways to help protect it from the inside. One is to use a spare VacMouse strip and lay it over the exposed bone. Another solution is to use a strip of a FoodSaver bag over the exposed bone parts.
As for length of time suit yourself but keep in mind that is a thick, large piece and then leaving the bone in besides means no aging from that bone side. Personally 21 days isn’t worth the bother and I would suggest no less than 45 days. Ron