do you not get your results electronically anyway? If you're still getting paper results could you not scan them and then ditch the paper? Any sample is only good for that moment it was tested anyway so by keeping them I suppose it just provides trend data. We are only sampling in our community patients every 6 months so for 12 months data it would only be 2 samples - not much paper as you can imagine! For main water plants being sampled monthly surely 2 years is more than enough to track the performance of carbon, softeners? Same for micro results - you may be able to see the condition of the membranes and ringmain starting to deteriorate.
To cut a long story short I'm not aware of any time period for keeping results. As we get our results electronically I will just keep adding to the pile as it's no extra hassle.
I suppose the main question to ask is what is there to be gained from keeping them - answer that question and you'll give yourself an answer as to how long you'd want to keep them. If there's an official answer out there then it must have been derived from just asking that question.