A coolant hose will typically wear out during the life span of your vehicle, with failures rising dramatically in the fifth year of a vehicle's life. The average four-year-old car has been driven nearly 100,000 kilometres, and it's about the time that parts begin to wear out.
The outside of hoses can be visually inspected for signs of imminent failure. Hoses can sometimes fail from the inside where weakened elements can't be seen. Therefore, replacement of the coolant and radiator hoses, should every done preventatively every four years, regardless of physical appearance. (truckers & bus lines do the same thing, scheduling replacement of key parts before key components break down, because it's cheaper than emergency repairs once the part actually fails, usually somewhere remote)
The materials now used in your engine's construction are different than those of years ago. Cast iron has been replaced with aluminum, plastic and brass. These materials, along with the coolant, create a rough battery with the hose conducting electricity through itself. This weak electrical current will slowly create tiny cracks in the hose tube, a pinhole leak or a burst of hose, which cause the hose to fail, in a process called electro-chemical degradation (ECD).
Cramped engine compartments result in hotter temperatures for hoses, which also increases the rate of ECD in hoses! In fact, for every 18-degree increase in temperature, the rate of ECD doubles. Heat can also reduce the overall burst pressure and cooling system performance.
The best way to check coolant hose for the effects of ECD is to squeeze the hose near the clamps or connectors once the engine is cool. Use fingers and thumb to check for weakness, not the whole hand, pressing within 5 cm of the end, where ECD is most likely to affect the hose. Check for any difference in the feel between the middle and ends of the hose. Some gaps or channels can be felt along the length of the hose tube, if weakened by ECD. Also, if the ends are soft and mushy, chances are the hose is under attack by ECD.
Vibration from rough-idling engines weakens both hoses and loosens their attachment points. Abrasion from sharp surfaces within the engine compartment can also rub through the outer cover, causing the hose to burst. While cuts and nicks to the outside of the hose, as well oil in the engine area which attacks the rubber on the hose covering, can cause a hose to fail.
You would be smart to replace a hose at the first sign of such wear, rather than waiting for hose failure.