How long does it take to break down polyester at the land fill?

It depends on the conditions. If you were hoping for a definite answer like "20 years" or something, sorry.

Ester linkages do hydrolyze in mildly acidic conditions, which can be reasonably expected in a landfill, so polyesters will, eventually, break down. How quickly depends on how acidic it is and exactly what form the polyester is in. In the form of small fibers... say, a shirt... it will degrade rather more quickly than if in the form of solid pieces like a soda bottle.