Well you would change the babies diaper when it gets wet enough to need changed or when they poop in the diaper.
If the baby poops in the diaper it needs changed right away or as soon as you notice it and some babies will cry and let you know they have a dirty diaper and need changed.
When the baby is wet it doesn't need changed right away but instead you can usually let the baby stay in the wet diaper for a few hours to allow them to wet the diaper a few times as least.
Disposable diapers draw the wetness away from the babies skin so they hardly feel wet in the diapers today.
So you do not have to change the babies diaper after every wetting otherwise you'll be changing lots of diapers.
When babies are newborns they'll usually need between 10 to 12 diaper changes per day so you should check the newborns diaper every hour if possible and change when needed.
As the baby gets older as in 1 year and above then you can check the diaper every 2 to 3 hours which is when older babies and toddlers need diaper changes.
If the diaper is just a little wet then it's okay to wait awhile to change it but if it's soaked you can feel it and then it will need changed.