Hey there Cath!
When I did it with Marshall, I would leave and not look back no matter what. So, if I put him down, sang songs and then kissed goodnight - and he would cry as soon as I put up the crib rail, I'd lie him back down, say good night and leave. Even if he was crying. Then as soon as I was out door with door shut, I'd start counting. 8 was our number. But lately, I've been counting as long as 20 if necessary.
I found with Marshall that after a couple of these, his cry would change, and I could tell the difference between ed and sad. Seemed with Marshall that he'd stop to listen if I was still out there when he was ed.
My rule was if the cry sounded irritated - like, I'm not getting my way - I'd let it go, until it turned into a real genuine cry. If it were quiet whimpering, I'd let it go too - because usually, he'd stop after a bit. I kind of stood out there and listened and when it sounded like he needed me, I'd go back in. Make sense?