I shoot a lot, and I'm getting ready to replace my string/cables after just getting the bow 5-6 months ago. If you don't shoot much, I'd say every two years is a good idea. Even if you don't shoot at all, these string materials break down over time. And yes, you have to do a complete retune when you change rigging.

But, if your bow is shooting like you want it then measure everything, tiller, brace height, axle to axle length, nock height, distance from your nock to your peep... centershot and prong height on your rest shouldn't change, but measure them anyway. When you get the new rigging, match those measurements as close as you can, shoot the string in (about 100 shots or so) and then twist everything back to your measurements. Cuts a lot of time off the retune.