If you have a less absorbent thinner adult diaper then you may need to change the diaper every 2 to 3 hours before you start leaking out of the diaper.
But if you have a thicker more absorbent diaper such as the Mega Max Diaper or any of the Abuniverse diapers then you could go 8 or 10 hours without needing to change the diaper before it leaks.
For hygiene reasons it's best to change the adult diapers every 3 hours minimum but for me when I wear thick diapers I just stay in them as long as possible which is around 10 hours sometimes because I love wearing wet diapers.