BirKineSS, Kelly is a little bit easier to find at FSH than Birkin. If you are looking for a Birkin, the best is to actually go there right when the shop opens (at 10:30am) I think. But for Kelly, I'd say any time of the day should be ok, but it might help to be there at 10:30am. Sometimes they have shipments in, in the afternoon too, so if you can't find what you want in the morning, you can try again later in the afternoon!
While you're in Paris, don't forget to try the Avenue George V store as well, if you can't find what you want at FSH.
Good luck with your Kelly quest!