What is the half life of plastic bags?

To preface this Q&A, different plastics and plastic thickness have different decomposition times. For example, plastic foam will never decompose.

I have seen some websites say it takes 10-12 years to decompose (which is hard to believe), another say 450-800 years, and yet another say 1000 years. Apparently, there is some debate about the definition of "decomposed": Plastic never fully decomposes. Over time it goes through a process of photo degradation and breaks down into smaller and smaller pieces.