I just replaced three 60 watt CFLs in my bathroom that had failed in less than 2 years. Box says 4 years for instant on minis.Its not the glass bulb, but the electronic ballast in the base of the CFL which fails from exposure to moisture.
Most consumer electronics contain semiconductor chips in plastic packages, which do not remain sealed over the product's life. This practice is good enough in dry environments, but hydrogen from moisture infiltrates silicon chips, eventually causing the transistor thresholds to change, which causes failure.
Depending on your local electricity rates, the expected life of a CFL is not long enough to recoup its cost in energy savings if the lamp is used less than two hours a day. Putting CFLs in little-used locations is likely also not energy-efficient considering the energy and materials used to manufacture and dispose of the bulb.
