OK, RN with a hubby who had valve replacement a year ago–sternum needs to heal up, not just because you might fall on it, but because you have to mount, dismount, and otherwise lift quite a bit (i.e your bodyweight) with your arms and upperbody for those tasks. So when lifting restrictions are off AND somebody can poke you in the breastbone without you flinching, you should be good to go. I would consider buying or borrowing a body protector just to give you an extra feeling of security.
When I fall I nearly always land on my butt. However, the worst fall I’ve ever had I landed smack on my right ribcage, and just recently managed to have a boob speared with a big old ugly tree branch when another big old ugly tree branch knocked me off my horse. Either fall couldve hurt a compromised breastbone. And a body protector, which I dont own one of, wouldve prevented either injury.
Mr Jeano ignored weight lifting restrictions everytime my back was turned and did fine, but he is a big guy with a huge chest.
Depending on your age and hormonal status, it might be a good time to get your bone density checked if you havent already done so.
I doubt if it will take as long as six months for you to be fully recovered. Three months maybe. In fact, there are people who have gone skydiving not long after open heart surgery, but they had special harnessess and chest protectors to make that possible, and were athletes in good shape otherwise.