It is like banjo22 said : the graves remain for a time of 30 years and you have to pay for the ground.
after 30 years the graves are levelled and used for next burial.
there are only few headstones with older dates than 80 years.caused by burial of grandfather and father
and than children in the same spot.
in very small villages eventually can be found older graves.
or if someone was an outstanding person in town, e.g. mayor