Mine lasted 6 weeks and I was told this is how long it takes bone to 'knit' together. I hope you never get the 'cracking' sounds. One evening I turned over in bed and there was a loud crack, just like someone treading on a large dry stick, breaking it. It was enough to wake up my wife and so it must have been loud because her snoring makes you feel like youre sleeping in the middle of a farm yard. I honestly thought I had broken my rib cage open. I still have no idea what it was and felt no discomforts.
You know you must avoid lifting heavy objects and I was told not to raise my arms above shoulder height for 6 weeks.
Quick answer, and it's best to wait for someone with more info.... It takes up to two years for the sternum to heal. As far as the clicking, make sure you mention this to your cardiologist, he may want to investigate it and take action. I personally probably couldn't handle knowing my sternum was moving by that much.