So, when you get fitted for the first time, you get assigned a centre and a clinician. The amputees would visit the clinic every time they need and at least 1 a year to verify that everything is ok.
Some very sophisticated limbs (like the ones with a microprocessor) require maintenance just like a car, minimum every 20 months.
The limbs require retightening, readjusting and sometimes regreasing, as it it a mechanical component.
But obviously, if the patient is feeling that something is not working they can attend their clinic as often as they need.