When does iPhone get backed up using iCloud?

Automatic iCloud backups should happen once every 24 hours. So your next one will be 24 hours since the last successful automatic or manual backup file’s time stamp. If the device is not plugged in and online at 24 hours, it should try again the very next time it is plugged in and on wifi.

The time it will take to complete the incremental updated backup will depend on the speed of your internet connection, and how much data has changed on your device since the last backup.

Simplest thing is just enable iCloud backups and then leave your phone plugged in and within range of wifi every night while you sleep.

It is usually 24 hours, but it can depend. Normally the iPhone has to be connected to wi-fi, power, and the screen locked longer than 2 hours, and it also can depend on the last backup. Understand, part of what I'm saying is just a guess as I cannot be sure, but it seems that mine didn't start last night until about 2.5 hours after I put it on the charger and went to bed.

As others have said, it isn't instant, and if you are more worried about instant, then you can create a new iCloud backup in Settings, but when you charge your iPhone overnight, like most people do when they sleep, it will backup daily like that. It is what I do and I get a fresh backup every night. My backup last night, was actually early this morning, because I didn't put my iPhone on to charge until after 11:00pm, and my backup was at 12:14am.

It is not instant. I would leave it on and see what happens every day. My phone backed up at 9:30 last night. I locked the screen about 7:30 PM.