Unless theres an electrical storm a router should last forever, telephone wires are not insulated. Nothing in there is under stress and most people leave them on, start up is where most wear occurs in a computer
More damage would occur where temperate varies a lot maybe if your house is not insulated well and the router gets to zero when the house is empty then 50c because you put the router next to the radiator. I imagine it'd last about 2 years anyway. Ours is 7 years old