That depends slightly on the nature and frequency of use. A heavy workload often leads to contamination and impairments that you may not necessarily see at first glance.
Whatever the case, you should proactively service your machines and not simply wait years for a fault to occur. This is not only much more predictable, but also much cheaper. It also lets you ensure the consistent quality of your welds as a reliable constant for you and your customers.
Serviced machines also give you the necessary security that you are complying with ISO 9001 requirements.