What is the best time to change machine oil?

I do agree with our friend Mohamed Deghmani ,s answer100%

the best time to change the oil in the machine is the one indicated on the manufacturer's instructions

You can do oil analisys, if anything abnormal found in that report then change the oil otherwise u can continue with it. Additionally u can save the cost. Refer OEM manual. If any breakdown happens inside the gearbox then its necessary to change the oil because of some debris in oil sump.