If the belt or chain breaks and you have an interference engine then you will have valve or piston damage.
The interval I have heard for replacement is 60,000 miles.
If the belt or chain costs $100 installed and you have an interference engine and the engine damage costs $1000 then you should replace the belt or chain when the probability of the belt failing gets to be 10%.
Because 10% of $1000 is $100.
If the belt or chain costs $1000 installed and you have an interference engine and the engine damage costs $4000 then you should replace the belt or chain when the probability of the belt failing gets to be 25%.
Only the car makers know the graph of
miles vs. probability
for these belts or chains and that's partially how they came up with the recommended replacement interval.
I'm not a fan of Clint Eastwood, but "Do you feel lucky?"
