the next night, I've found that the wait is about half. Then the following night half again.

Remember the following:
1) SHE IS FINE
2) SHE WON"T REMEMBER this torture you're inflicting on her.
3) Crying is absolutely exhausting. Even more for her than for you. She cannot physically keep it up indefinitely
4) From an evolutionary standpoint, crying exists so that children don't die when they're young. Crying is so distressing it sort of forces you to respond, protect and fix. It's a baby manipulation! Once you consciously know that the reason the crying is so distressing is a psychological trick God and nature put in your brain, it's a little easier to suppress.
5) Imagine a 4 month old sleeping through the night. It's worth some terrible nights.
6) DON"T GIVE IN!!!!!!!!