You can always come to the monthly SBCK meeting. If Dave has his meter with him, you can measure your blubs out-put. Might get a little longer use out of those bulbs, than the recommended 6 months.
Or you could pick up your own solar meter and periodically check you output. I think Dave had mentioned in one of his posts about getting one. I have also read somewhere on this forum that some of the bulbs will last up to one year. Not sure though.