As said in another thread - it takes usually 3 months before you are over it.
But you can start kitesurfing after a few weeks with a seat harness, so dont worry about it
Rest dont really help, just avoid "pain".
But dont try to use a waist harness, as eventhough you might not feel any pain when you start, you will suddenly feel very bad, and it will be even worse than just after you got the rib injury.
So just go out with a seat as much/soon as you can
It is a very common injury, and can happen with many different harnesses.
After some months, or 3, you can use a waist harness again - depending on how "serious" your rib has been hurt.