Hi famoussenior,
I have played golf for close to 40 years (a few of those years professionally) and I have had 2 surgeries worth of experience. Everyone heals at different rates but the "average" time for the sternum to completely heal is around 12 weeks. I started chipping and putting at about 6 weeks as this is when I was allowed to drive. I drove up to the local driving range that had a short game practice area. By chipping I mean only chipping right around the green not pitches or bunker shots. At about 10 weeks I started hitting some half swing wedge shots (maybe half a small bucket) and did more chipping, pitching, putting. At 12 weeks I started taking some full swings with my wedge and some [very] easy swings with my irons and woods. From that point it was just a matter of not having pain as I started to add more power to my swing. I think by around week 12 its more a matter of how it feels to you. If you take a full swing and it causes you pain you are not ready yet. But more than likely by week 12 you are not going to do any damage to your sternum. It will let you know whether or not it's ready.