How long can LO stay awake

We just started to sleep train for naps. LO is 4 months exact. I am laying LO in the crib wide awake then sitting next to the crib and shushing and hand on stomach as needed to sooth. We have it as dark as possible with a sound machine on.

First attempt of the day she didn’t fall asleep at all, so we got her out of the crib, did a bottle and 30 minutes of play. Then we put her back in the crib. She hasn’t slept in 4 hours.

My goal is for her to fall asleep on her own in the crib and as she gets better at this, I will leave the room eventually.

My question is how long can your LO resist sleep before eventually passing out.

This seems cruel and it’s very hard, however with me in the room, she has done very minimal crying.