It really depends on the amount of use. Every 2 years sounds good. My rear usually fails in that amount of time. If you are looking for durability, sealed cables are known to last a long time.

How often should shifter cables be replaced?

Registered. 5000 to 6000 miles is a good time frame to replace the cables. Other wise wait till they brake and either have a good stiff pedal in or nice easy spin, depending on the broken cable.

Can you reuse shifter cable?

You can reuse the existing cable/housing BUT the cable will "thread" through the shifter as opposed to brake levers where you can just remove the cable from the slot in the lever and leave everything in place.