Answer:
most plastics break down faster in the presence of sunlight with the exception of some UV resistant plastics if you left it in the sun i would venture to guess that it would last about 7 years but if it was burred(in a the dirt) i would guess well over 100 years but if it was in a land fill i think that some of the acids and chemicals created by the decay of organic materials (ex banana peals and the like) i think it may speed the proses i think but it raises the questions that after it is broken down is the chemicals that make up the plastic still there.... a fun story i once seen a plastic 5 gallon pail ant it was half under ground and half in the sun after about 3 years the in the sun part was very brittle ad crumbly you could crush it wit your figures but the burred part was still in new condition. i hope this helps but i am not a professional in plastics i base this on observation