A toddler will need their diaper changed at least every 2 hours. You can feel the diaper on the outside to see how squishy it is.
If the disposable diaper is pretty squishy then it's time to change it. If it's not all that squishy then it's probably okay.
With poop you will know it's time to change your toddlers diaper as it will smell really bad. If your toddler poops the diaper should be changed as soon as you notice it or if on the road in a car change the poop diaper at the next available stop.