Record Keeping Question

For those of you who home school... how long do you keep completed work? We use a completed drawer and completed file system where completed work is placed in their drawer and I gather it for grading. When  I have graded and recorded the work, it is placed in a completed file. Some things are placed in the kids portfolio, especially if it is a basic skill that I want to track progress on (i.e. writing samples, reading comprehension, certain math skills).  At the end of the year, we end up with really thick files that I tape close and label with the year and grade level and placed in a large storage bin downstairs. After 8 years, I have lots of completed files. I live in a state that requires no testing or paperwork, but want to be sure to have a good records of the kids schooling. Thoughts?