The average life expectancy of a belt is around 50,000 miles for most belts. 

Hoses, on the other hand may not show visible signs of weakening on the outside because they will usually break down from the inside.  Engine oil, transmission oil or power steering fluid leaking onto the outside of hoses, however can cause them to weaken.  A good time to replace radiator hoses would be when replacing a water pump or radiator coolant.  Its best to have a trained technician periodically check the belts and hoses to assure there is no internal weakening or external fluids dripping on them that could cause a failure.