For a road bike used by a recreational cyclist, what is the life expectancy of cable housings for shifter cables, either in terms of miles or time?

I ride throughout the year. I don't ride in snow or ice conditions and do not ride in the rain unless it starts raining during a ride.

I have two road bikes, one with 12,500 miles and the other with 14,000 miles. I've changed cables as needed but not housings. The bikes are kept inside my dwelling when not in use.

I would say that the answer is very open ended and doesn't even have a rough estimate. Depends on so many variables. Storage of bike, quality of cable housing, how the person shifts/brakes, how much the housings are flexed, is the bike cleaned after bad weather?

I can see a 10 inch piece of brand X housing used on a downhill MTB going though mud and everything else failing after a few months whereas that same 10 inch piece used on a comfort bike kept indoors might not fail for 10-20 years.

If your shifts are crisp, I'd forget about it and just ride. It sounds like you keep on top of your cable maintenance and so you should notice when it's time to replace the outers.

My experience is the housings go before the cables. They get brittle and don't flex. Mine last around five years and aren't all that dependent on mileage.

It's highly dependent on your individual situation.

For me, a rear cable lasts around 2500 miles. My current housings have 2.5 years and 18,000 miles on them and they seem to be doing fine. It's hilly around here and I shift a lot which is why I have to change the rear cable so often.

Housings last a very long time for me. It took me 15 years to get to degraded shifting from my Campagnolo housings. There was some rust on the ends under the ferules although prior to the rear housing getting sticky the cables still ran smoothly with a little oil when the cables were replaced. The rear derailleur loop cracked sometime in the last 5000 miles between when I took the derailleur apart to clean and grease it's sticky upper and lover pivots and the housing/cable/etc. replacement.

It feels like I'm always replacing right shifter cables, although I started keeping track of these things and it was 18 months and 5625 miles before it broke strands at the shifter end which degraded shifting and speared me in the finger.

I replaced the front brake cable every time I rebuilt my right shifter which was about twice that interval but have yet to see a broken strand.

I run Campagnolo ergo levers - first generation until the recent rebuild where I moved on to Ultrashift.

I run tight cassettes (currently a 14-23 straight block, 13-14-15-16-17-18-19-21-23 before that) and shift like I have ADHD.

I don't avoid rain and park in a garage or shed. When I lived in Boulder, CO I built another set of wheels and left a set of 27mm cyclocross tires mounted for snow days.

With STI or Ergo shifting it is best to change housings every time cables are changed. This is especially true for the short housing at the rear derailleur. Housings are susceptible to collecting dirt and corrosion causing excess friction on the cables. The short housing is closest to the ground and picks up dust and dirty water spray more than the other housings.
I buy my housings and cables in kits from Shimano and Campagnolo. The short housing is pre-cut to the correct length.

There is really no way to inspect cable housings so staying on top of the shifting performance of the bike is the best way to detect trouble. Balky or hesitant shifts, especially to smaller rings/cogs which are spring-driven, or a sudden need for frequent adjustments would be clues that something is amiss.

Check the inner wires visually where you are able to; fraying or broken strands at either end are trouble indicators, especially inside brifters. Catching a failing wire before it breaks will save you a world of hurt; getting broken wire ends out of many brifters can make a grown man cry; it is far better to catch it before it breaks.