Well This thread has gone all over the place. First I've been told I need to change the cable and housing annually no matter what according to cxwrench. Which is complete bunk.
Then you say "mileage is a key variable" - To a degree this would be correct.
And now we have drifted to "it depends on how frequently you shift and whether you live in a flatter or hillier area - This is probably the biggest key variable.
Seems to be a lack of consistency here is all I am saying.
You don't seem to ride any one bike very heavily -- esp your road bikes, which have the types of shifters we're discussing.
Mountain bikes get used the most, but the road bikes have more mileage on them. I take longer rides on the road bikes than I do on the mountain bikes.
We've come to an impasse and will have to agree to disagree. When and if I start to notice poor shifting, I'll change the cables and housing. But I know with 100% certainty that it doesn't need to be done on an annual basis or biennial basis in my case. That's just throwing money away.