it will depend on from where you are getting the cat5e cabling. some companies manufacture with lifetime guarantee, and some other display their product as a limited guarantee/warranty. well these are all terms to say that the first one with lifetime guarantee will last quite long. the second one says it wont last that much. ideally once installed a cat5e cable should last atleast 5 years, that is the bare minimum. if present in an non - abrasive, non - corrosive, dry area where there are no sharp objects, or the cable is not being disturbed by further installations, or people treading over it, a cat5e cable should last for much more than 5 years.
Arani's spot on. The cable will last for a ridiculously long time if it's left alone. Cables hardly ever go faulty; they get broken. But the other side to how long the cable will 'last' is how long it remains useful before the world moves on and your Cat 5 cable becomes useless because you now need Cat 17 to support Terabit ethernet.
Okay, so I'm exaggerating a bit but the point is valid; the rate of change of the technology coupled with how leading edge the activity in your building needs to be will determine the effective life of your installation