Units I have been under normally refresh their IT assets (computer towers, monitors, printers) about every three years.
The only reason it sounds that they do it is because of a warranty expiring. Many computers can be ugraded if need be and CSA's should have knowledge to replace/repair parts. Refresh for any IT assets should be five years or more.
To save additional time, money, and manpower, the entire military should move to computing over IP. It takes away all troubleshooting issues to the end user. It moves all information to a seperate server and allows for safer data processing as no data is sent to the user. USB can be locked down giving the end user no capabilty of renenabling it.