What is a file?
http://en.wikipedia.org/wiki/Computer_file

And where do they live?

Today files live on flash drives, smartphones, tablets, compact flash, SD cards, hard drives, solid state drives, optical discs, tape. And now they stay in reach via the cloud.

How long do they last?
That's a tricky question to answer because it depends on a few factors:

  1. Can the file be found? Does anyone want to find it?

  2. Has it been catalogued in some manner with metadata?

  3. Is the file still readable by an operating system and application?