Bearing Replacement?

Hello all, I am operating a 3-Axis CNT Motion 1000 series CNC machine, and besides the normal maintenance we do on the machine, we are wondering if there is a certain time period for replacing the linear bearings on our Z and X-Axis'? We have had the machine for over 5 years, and besides replacing the Y-Axis bearings about 9 months ago, and our timing belts a couple of months ago, we haven't seen any major wear and tear on the other bearing elements. For reference, our machine self-lubricates these particular bearings, so as long as we keep the oil reservoirs full, and regularly clean the dust out of everything, they all work just fine. But as we didn't really have any sort of reference about these particular parts from the machine manufacturer, we thought it might be prudent to ask around. And if there is a need to replace these parts, apart from hiring someone, does anyone have any links to any procedural videos on performing this task? We have access to a forklift to hold up the larger elements (the way we did when the Y-Axis bearings were replaced) if needed. Thankyou for your time and any help you can offer!