Ripping the band-aid off and trying CIO, baby has been crying for 25 minutes

It's so hard to listen to him crying, but I'm also so tired of fighting with him every night to get him to sleep.

He's 9 months and still protests being put down at night unless I nurse to sleep (which sometimes doesn't work anymore) or rock him just right until he falls asleep (which lately seems impossible).

Any advice to deal with the crying? Does this get easier? What if he just cries for hours, do I call it at some point?

I still feel the need to watch the monitor just to be safe, but that makes it even harder to let him cry.

EDIT: I turned my monitor screen off for a bit, put on Trolls Band Together and pulled out my coloring books - he fell asleep sometime between 1 hour 10 minutes and 1 hour 30 minutes. It felt so long! But hopefully it gets better, I'm going to commit to trying it for a week and maybe try to lengthen his last wake window and see what happens.