Answer;
3 years
This may vary in your state, but S/P2 recommends keeping all environmental documentation a minimum of 3 years.
Step-by-step explanation;
The major benefits of environmental record keeping are accurate crediting of manure nutrients applied to cropland and improved management of manure storage areas for protection against accidental discharges. Good environmental records can provide protection in the event of an extreme weather event or accident and can aid in verifying that your facility meets its planned best management practices.