A child who is potty trained should be able to hold their urine for at least 30 minutes to an hour without wetting their pants.
However even if a child is potty trained and able to hold their urine they should go to the bathroom and pee in the toilet and not hold it longer than 30 minutes.
Holding in urine for too long can lead to Urinary Tract Infections, Bladder Infections and other health issues.
For a child from birth to 12 months of age it takes around 1 hour to fill their bladder and for a child of above 12 months to age to 3 years old it takes around 2 hours for the child to fill their bladder.
For a child above 3 years to age 12 it can take around 4 hours for a child to fill their bladder.
As a child gets older they can hold their urine longer and will usually pee less often so they might keep their diaper dry longer and may not need a diaper change as often.
When a child is 2 years to 4 years old they may only need a diaper change every 3 to 4 hours and they might hold their urine in and let it all out at once at a certain time of day.
So if the child is going 3 to 4 hours until they wet the diaper then they are likely ready to use the potty and can hold their urine in.
I believe a child really is not ready to use the toilet and be out of diapers until they can hold their bladder for at least 3 hours and then wet their diaper.
The child also should be able to pull their pants and underwear down themselves and make it to the bathroom.