there is not a set date to which it should be replaced, but if you flip it over it should not be cracked or frayed. if it is its time to get a new belt.
Whenever it gets dirty.
every 3000 miles
Every 12 months.
5000-7500 miles
it should be checked every 3rd oil change 5-10000 km
Change the fluid and filter every 36,000 miles.
Change it according to the manufactures recommendations as listed in your owners manual.
if your car's belt has bits of fabric coming out
Every 15,000 miles.
Every other oil change, drain water valve once a month.
I would say at least once every 3-4 months.
Every 4000 miles. Change it when oil is changed, along with oil filters