Cables don’t “wear out” in the same way that some components, such as vacuum tubes, do. In most cases, unless it is damaged or suffers wear from being coiled/uncoiled, stepped on, pulled, twisted, bent, or otherwise abused, a cable will continue to perform just as it did when you purchased it. You can infer from this that cables used onstage for live performance probably won’t last as long as those used to “permanently” wire up an equipment rack in a studio.
Another aspect of this is that a poor quality cable can often be the weak link in a signal path. Therefore, another reason to replace a cable is to upgrade it to a better one to improve signal quality and cable durability. In other words, inexpensive cables may degrade the signal enough to sound noisy, lose signal quality, or flat out break sooner than high-quality cables. There’s no exact number of days that a given cable will last, but in general if you’ve bought the cheapest cable available, you can expect it to need to be replaced at some near-future point, while a more expensive cable might last for many years making it a better investment in the long run.
So, having said that, you should replace your cables when they stop working, become intermittent, or you notice noise or signal degradation that can be traced to your current cables.
And the unspoken moral of the story is: always keep spare cables on hand to save the day when a cable fails or starts to go bad!