The length of time it takes for a diaper to decompose is an estimated 500 years.
However the actual length of time for a diaper to decompose will depend on whether it's in a landfill, out in the sun and elements or just buried in the ground.
The plastic outer layer of the diaper takes around 20 to 30 years to decompose and the absorbent material takes around 100 to 500 years to decompose.
When a disposable diaper is left out in the heat and sun it will often decompose sooner and may take only 10 years or less to break down.
Some diapers from the 90s have been found in the landfill that were still nearly fully intact after around 25 to 30 years.
Other than the dirt and old urine and mold it looked the same as it did when it was sent to the landfill.
Many of the disposable diapers today will be around 30 years or longer and will likely outlive yourself, your kids, your kids Grandkids etc.
That is unless you burn them and get rid of them very quickly.
If burning disposable diapers you should remove the SAP inside of it that is wet so the rest of the diaper will burn better.