We moved into a newly constructed home (almost 4 years ago) and we switched out the recessed (and nearly every other light) for CFL almost immediately. Two recessed lights over the sink in the kitchen caused the temperature to rise 5°F in that area, which was enough to realize just how much heat they were giving off - let alone the difference in energy use. I've saved the incandescent bulbs to put back in the fixtures should we ever move. I'll take my expensive CFL and LED lights with me.

You may also want to consider LED lights for some of your lights, recessed lights and lamps, which use even LESS electricity than CFL. Sam's Club now has several styles of LED lights that are very affordable.

We like LED lights because they come on with full brightness immediately (unlike the horrible pink glow from CFL until they warm up in a cold bathroom), and aren't affected by on/off use like CFL (which shortens the life of the CFL bulbs). LEDs don't contain mercury (you can safely use LED lights in children's rooms or in lamps and not worry about broken lights and mercury, and last longer. In our bathroom we have one recessed LED light and over the double sinks we have 4 LED lights and 2 CFL in order to have sufficient light.

-Grainlady

From a purely mathematical standpoint, you should replace them as they burn out. The payback on CFL's is still longer than the average life of a standard bulb, so that is the cheapest option. Also, the price of CFL's and LED's seems to be going down and down. If your standard bulb lasts a year, the CFL you replace it with in 2011 will probably be cheaper than if you swapped it out in 2010.

Of course, if you put some non-monetary value on using less energy, then you should swap them out earlier.

I'm beginning to get turned-off on CFLs in recessed can fixtures. I've had FIVE go bad prematurely in the last few months, two were just a couple months old. I think the ballast heat can't dissipate properly and reduces the lifespan.

we started swapping our old incandescent bulbs for the CFLs a few years ago when they where still very expensive so we did it as the old ones failed because they were just too expensive to buy in bulk. here in oz they dont sell the incandescent ones anymore and the advice from the "experts" is the energy savings with the CFLs outweighs any savings from using the old ones until they burn out. IMO though if you dont want to change over all your bulbs you should change out the bulbs you use the most because you'll get the quickest return on energy savings with those

In our old home, I figured that we used the kitchen, master bedroom, and master bath cans the most. As one can light would go out, I would replace it with a CFL. If the burned out bulb was in the dining room, though, I would move an incandescent light from, say, the kitchen, to the DR and install a CFL in the kitchen. I eventually got all the can lights changed to CFLs except the LR -- we had a dimmer on those lights. And then we moved. So far, after 9 months in our new home, not a single can light has burned out. I except to start seeing a few soon, and I'll do the same thing with changing them.

I just bought CFLs at Home Depot for $3.99 per 4 pack. Target had some on clearance, 7 pack for $2.88. I've been very happy having converted to nearly all CFLs now in my house, indoors and out. They do vary though. Some of mine are instant on and others take a fraction of a second. I have two on my kitchen peninsula and can see the difference, one is on immediately. But it doesn't bother me. Most of mine are all nice cool white and I actually use a smaller wattage comparable for reading than with incandescents. I noticed a significant difference in electricity after replacing just 3 high-use lamps and that sold me on them.

Payback is going to depend on how much you use them. Seldom used and little use, for instance a closet light or something, you might want to consider whether it's worth it or not.

I have them outside as said and the cold weather doesn't bother them. Yes in frigid temps they might take 15 seconds to come up to full brightness but that's not a problem. Plus if I leave the outside lights on, it's not that big a deal.