You should be able to get a year out of one of the screw-ins. I know the CF tubes will go as long as 2 years and still work well.
As a reference, what I do with the screw-ins is I buy 2 bulbs and after 8 - 10 months I take the old one out and screw in the new one to see if there is any appreciable difference to my eye. When doing that, I've gotten over a year out of mine before I thought the new one would do a more efficient job.
Len