Revalidation of DM water generation System

Dear Sir,

I have a query regarding the revalidation of DM water Generation system.

we have installed the system in 2012 and till now it is working fine and at the time of installation, performance qualification was conducted and got the desire results.
Apart from this, we don’t have the specific SOP for the revalidation of DM water plant.

Pls confirm what is the revalidation criteria if no change in the system and system is working as per requirement.

Pls also confirm if we will conduct the revalidation qualification, how many days / period required to conduct the analysis.

Regards,
Ravi Yadav