No one right now really knows for sure how long it will take for disposable diapers to decompose but the estimation for a disposable diaper to break down in a landfill and decompose is from 200 to 500 years.
You'll be dead, your child will be dead, your grandchildren will be dead and likely your grand children's children will be dead long before your own child's disposable diaper breaks completely down in a landfill.
So yes that's a long time.