Rubber and fabric belts deteriorate for 2 reasons mainly, the amount of flexing suffered and the age or the belt. A 3rd reason my or may not be present, contamination by liquids, not usually present, and heat, always present.
The belt change periodicity is rarely stated in makers handbooks, if you query the main agents you will get hugely differing answers, so you have to work it out for yourself.
Does the belt drive a camshaft? If NO, you need not be so particular about regular changes, as breakage is not going to cause valve to piston contact. See Ford Escort Diesel pump drive belt.
Even if it does drive a camshaft, some engines don't suffer piston to valve contact due to careful design, such as the basic Punto Fire engine.
Does the one belt drive both water pump, camshaft and injector pump? This is a heavy loading and more frequent change is indicated.
Is the belt a wide thick one or does it appear to be thin and flexible? The thicker the cross section and width, the longer the periodicity one might assume.
I asked the service manager of Fegimotor Almeria for the recommended change period for our Jeep Limited. He replied 140,000 kms. Other makes suggest half to one third of that figure. Some makes do not admit their engines have a drive belt at all.
Our Jeep has 30,000km showing but is over 10 years old, and the belt has never been changed from new. I am not yet alarmed but intend to change it before the next annual test.
To sum up, between 4 and 10 years and between 40,000km and 140,000km. Not easy to make sense of, but batter safe than sorry.