After having done some research about fluorescent lights used for plant growth it seems that you should change the tubes every 6-9months. At first a screw in CFL
appears to be more expensive since you would be changing the internal ballast every time as well and not just the tube, BUT with the old tubes I can't think of too many uses for them (undercabinet lights can only be used a few times before it starts to look really silly having them lining your house); on the other hand the screw in CFLs would still provide plenty of light for people to get by in their day to day activities while saving electricity over incandescents. A quick count revealed that my smallish 3 bedroom house has about 35 screw in fixtures and lightbulbs. By the time I fill every fixture in my house from aquarium leftovers I probably would have burned out the first ones I installed. This way I am not wasting money on throwing out old tubes...I am saving money on electricity. Additionally, from what I have seen at pet stores and hardware stores (home depot, etc) screw in CFLs rated at 5500K to 6500K are coming down in price to about $3-4 each for 13-14W bulbs.