From experience, I have seen GMP documents get updated fairly often and the reason for them being updated is usually because the previous instructions were unclear and created confusion. I have also seen GMP standards updated for technology updates, or building updates, such as a wash room being moved, and as a result, different tools were needed to fit the new hose requirements, etc. One company that I worked for, we would review GMP SOP’s every 2 years to determine if any needed new revisions or changes to either keep up with updates or to clarify them.