Disposable diapers take around 500 years or more to break down in a landfill.
However that is just an estimate as we don't know for sure as disposable diapers have not been around long enough to know for sure how long a disposable diaper will take to break down and decompose.
Disposable diapers are made of plastic and SAP which takes a long time to break down but eventually they will break down.
But on average it takes an estimated 500 years for the disposable diaper to break down.
That would mean that the disposable diaper will still likely be around for several generations.
You could compost some of the diaper such as the SAP and then throw the plastic part of the diaper away.
Or you could burn the disposable diapers.
I burn my babies disposable diapers to get rid of them.
In the winter I throw the disposable diapers in our wood stove and they burn down just fine.
In the summer and spring months I burn the disposable diapers in our burn pit.
However I throw them in the burn pit and allow them to dry out for a few months in the hot sun and throw other brush, trash etc on top of the disposable diapers to help them burn.
Sometimes it takes a few tries to burn the disposable diaper completely but they eventually burn up to ashes.