2 years...one of them I know is at least 4.
My new Walmart Phillips ones are noticeably dimming in months ... They are also lighting slower by the week. I find the long 40W tubes much better for everything.
There are no dimmer switch versions, our house is set up with about 25 ceiling cans with dimmers on the the various circuits. Makes CFLs useless for most of our lighting.
I think the Congress and their Soviet style central planning mentality should get out of the planning business. Market forces are far more efficient for everything, from healthcare to light bulbs. The end result with the Soviets was there weren’t enough bayonets to force anyone to go out in the fields and pick potatoes.
In the end, I think most will give up on mercury laden CFLs and switch to LEDs. I don’t plan on buying anymore of the CFLs.