I update it whenever the "nag screen" shows up.
Then again, I do not have too many books either that I can't lose. Most of my 467 books are from a CD that came with a computer magazine.
When downloading from Feedbooks, Smashwords or the like, the books normally goes straight into the Kindle library and not via Calibre.
Only a few books or blog contents have been through my Calibre for converting to Kindle.
(In fact, I might just throw out all the free books from the CD and start with a "clean" library from scratch.
)