I guess the answer would be zero in my house because I honestly can't remember the last time I changed a light bulb.
I was reading an article in the latest issue of Popular Mechanics about some tests they ran on light bulbs to determine if the new spiral low wattage kind are as bad as every says they are. They even have some anecdotal story of some old man who bought two cases (240 bulbs) of the old incandescent kind to hoard away. As expected, the Popular Mechanics tests pretty much showed that you are crazy not to be using fluorescents.
To the best of my recollection and sifting through past posts, I'm guessing that I made the switch all in one evening more than five years ago, probably closer to six. I had some three dozen light bulbs in my house, thanks to the previous owners like of ceiling fans that have four bulbs a piece, and I changed all that I could. I had a couple nightstand lights and a small chandelier that had oddball sized lights that I couldn't change but they were all low wattage anyway. I went from changing at least a couple light bulbs a month to replacing only one bulb in almost six years. The rest just keep on ticking. Assuming at best case that my old incandescent light bulbs lasts two years and cost me a fourth of the cost of the fluorescent ones, they have almost paid themselves back just on longevity alone and more so if you count the fact that the fluorescent ones also use a small fraction of the energy. Nor does it count the energy saved by not having to cool your house more in the summer to counteract all the heat that the old ones put out. Remember those easy bake ovens?
I don't think I could go back to those days of ancient technology. In fact, when I finally live out my dream of building my own house, I will not put a fixture in place that can't use fluorescent bulbs and most likely will mostly use fixtures that use LED bulbs which cuts the energy cost of fluorescent bulbs by another 50%, last six to seven times longer and put out even less heat. Instead of hoarding light bulbs as the man in the PM article did, I will hoard something more dear to my heart like books printed on actual paper.