A cable modem should easily last from 5 to 10 years or in some cases even longer.
Make sure you keep the dust off the modem and prevent the modem from overheating by keeping things off the top vents of the cable modem and it should last a long time.
But then again it may not last as long if the lightning strikes it or a power surge gets to it.
But you shouldn't have to replace your cable internet modem very often.