First of all, I don’t think you should wait for something you really want, you should work for it. Make it happen. Don’t wait for it to happen.
But if you have to, just don’t wait forever.
It all depends on what it is you are waiting for. If you are waiting for someone, you need to understand that you may wait forever. If that person makes you wait for them, just move on.
Pretty much everything else you could possibly work for. But make sure you actually want it.