Plastic does not biodegrade and when buried in landfill will be around for several thousand years.
The Australian Bureau of Statistics reported 1.9 million tonnes of plastic was sent to landfill in 2010-11 (ABS 4602.0.55.006-Waste Account, Australia 2010-11). Whilst plastic recycling helps, for the same period less than 288,000 tonnes was recycled (PACIA 2010-11 National Plastics Recycling Survey).
Sadly in Australia, for every 1 tonne of plastic recycling there is 6.6 tonnes of plastic sent to landfill.