Again, I've never heard that, even as a rule of thumb. Outside of California (I won't even get started on that state), what reason would there be to retain them more than 3 years after use? If the company that manufactured the chemical has no requirement to keep them, why would we?
They are required.
It is for employees, past and present, who have worked at your facility in proximity to the chemicals or with the chemicals. If those employees develop some illness or health issue that points to that chemical as the possible cause you could find yourself in deep doo doo if you can't provide an SDS or MSDS. If the manufacturer of that chemical is out of business and you can't get an SDS or MSDS, then what would be your position at that point?
Yes, may seem far fetched, but could be reality. So why not hold onto them? Even if you scan and file electronically it really doesn't take that much time and effort, or space for the potential of deep doo doo lawsuits and liability.