Cam belts should be replaced following two principles.
Mileage and time.
Even if the mileage is low rubber/composite belts degrade over time and cars that sit idle stress the belt where curved more tightly in the same places as a piston compression stroke will be where the engine stops. Which is why starter ring gears wear in specific places also. If left for long periods (especially outside) the bright steel cogs get a film of rust and this contaminates the belt at those places too.
When starting an engine a massive shock load is taken by cam belts which is why many snap at that time and for reasons mentioned previously they will snap at specific points. Once running its a linear load.
Low mileage, rarely used cars should take a year off any timescale for replacement and I would change them every three years at least and would always recommend Gates (they invented the serpentine belt)