A toddlers diaper needs to be changed at least every 2 to 3 hours and right away if they poop.
Toddlers tend to save up their urine and then let it all out at once and they usually start to urinate at around the same times during the day so you can usually tell when it's time to change the diaper.
Leaving a wet diaper on your toddler for too long can lead to the urine turning to ammonia, and causing rashes, and UTI's.