How long does it take for a landfill to fill up?

Landfills are not built then watched until they fill up. An engineered landfill is sized so that it fills up in an estimated period of time. Usually the interior is divided up into cells which are filled sequentially so the status of the estimate can be corrected.

Building against an estimated lifespan allows planning for the development of a new site to be done in an orderly fashion.