I calibrate ours, it is on a 5 year schedule, it all depends on how much it is used. We have several smaller ones that are calibrated more frequently (2-3 years), the big table is used only by me, so I can extend it some. If high accuracy measurements are not required, you can use a portable cmm to validate it for flatness, at least to a degree. Like other measuring equipment, there is no sense calibrating to ten digits if you only need two. Establish your need first then apply the necessary calibration.