Straight forward question. One bulb in your two bulb ceiling fixture goes out. Do you replace both while you’re on the step stool? Or just replace the bad one? Or do you wait until the room no longer has any light and then replace both burned out bulbs?
Yes, I still use incandescent because I prefer the light they give off.
Just the bad one. No telling how much life is left in the other. Why waste bulbs?
I would just replace the bad one, but there’s probably a valid reason if you’re up there anyway to replace both. The rated hours are usually well defined from a normal distribution curve, but sometimes outliers on both ends occur, and it’s possible you have an early failure or a late failure, or one of each.
It could be frustrating if the bulb you didn’t change burned out the very next day. 
But, there’s really no way to predict how much life a bulb has. But it does make sense that if they are changed in pairs then they should burn out at very similar times.
I used to routinely change both because light bulbs were dirt cheap. But, now I’m using my stockpile of incandescent and when they are all used up, then I’ll be forced into using the new bulbs currently sold. I’m probably fine for at least 10 years.
I’m a lazy bastard. Unless it is in a room that requires the maximum light I wait until all of the bulbs in the fixture are burnt out.
I’ve waited a month for the 2nd bulb to burn out. I guess it’s a survivor. 
I pulled out the step stool and changed just the bad one a few minutes ago.
For the moment, I have both working.
I even went to an electrical supply house a few years ago and bought 130V bulbs. They are supposed to last longer. Short spikes in the power grid won’t zap them like a 110V bulb.
How hard is the fixture to get to? If I needed a special ladder & tool & three friends to get to a fixture high up in a stairwell then I’d sure change all of them. Or if the fixture is delicate and difficult to safely get apart, where each time I do so I’m risking breaking something irreplaceable.
Conversely, if the bulb change is easy there’s IMO no reason not to run the remaining good bulb(s) to the end of their natural lives.
Replace both with 13W CFLs (800 lumen). They cost less than $2. each and last for more than five years so that problem won’t reoccur for a long time. Also use 75% less energy while providing the same light output but producing much less heat. Many people are afraid of them because they contain a minuscule quantity of mercury (.5 mg) but never think of their wall thermostat which has ten times as much mercury (5 mg) in a little glass vial. I’ve run 48 CFLs in my home for years and never broken one.
Why no option for wait until both burn out then only replace one of them?
ETA: Should be an option for wait til both burn out then curse the darkness too.
Timetrvlr: And these CFLs work with all my existing dimmers how?
With only two, I’d probably go ahead and change it. With more bulbs, I start to let it go for a while until it gets too dark for my tastes.
I’ve never heard of changing all the bulbs at once. Bulbs are not in any way consistent with how long they last. I guess I might if it were hard work to do it, but usually I can just reach up and do it, maybe standing on my toes if the fixture is really close to the ceiling.
Just the bad one. I assume that last time, I just replaced the bad one, which is now the good one.
To me, it’s a coin toss.
On one hand, both bulbs get the same amount of usage. If one burns out, the other is just as old, right? On the other hand, if you wait until both burn out, it might get tricky having to handle a flashlight while you’re changing at least the first bulb.
I think the bottom line is how difficult/tedious it is to change the bulbs. Once I got tagged to replace the aircraft warning lamps on a 1,400’ LORAN transmitting tower. Equipped with, I might add, automatic bulb changers. I changed out every bulb on that ascent. The bitch of it was, I wasn’t allowed to just toss the burnt out ones, I had to bring them down with me.
I’ve changed a lot of light bulbs in my day, and I never remember the other bulb in a two bulb fixture going out soon after.
We redid the lighting in our kitchen so we have about a dozen bulbs in individual fixtures. They all got put in at the same time, but there is a wide distribution of when they burn out, so light bulb lifetime is a one or two sigma distribution at best.
Just the bad one. We’re talking something accessible with “a step stool” and not something requiring ladder levelers, scaffolding, extension ladders, professional high-ass shit people or anything more complex than the couple of minutes required to carry a step stool or step ladder into the room, here, right?