Here’s an excerpt from a great article about how switching CFLs on and off can shorten their life and how long you should burn your CFL.
“Switching CFLs on and off does shorten lamp life, but [the] conclusion that they need a three- to five-hour on-cycle to maintain a reasonably long life does not appear to be correct. Robert Clear, a staff scientist at Lawrence Berkeley National Laboratory, told Enviromental Building News that it has been difficult to get data on this question, but a 1998 study of electronically ballasted CFLs found a 20% reduction in lamp life if the on-time was reduced to one hour. With significantly shorter on-times, the lamp life is dramatically reduced: with 15-minute on-time cycling, lamp life dropped 70% and with five-minute on-time lamp life dropped 85% (which brings the lamp life close to that of incandescent light bulbs). “This suggests that you should consider replacing incandescents with CFLs in any application where the lamp is on an average of [at least] about 10 minutes per start,” said Clear. He added that “every switch cycle is equivalent to about 6 minutes of lamp life. This means that you should turn a CFL off if you think it won’t be turned on again for another five minutes or so.” This approach should also maximize electricity and cost savings.”