How often should I recaulk the ceramic-tiles in my bathrooms and kitchen?
Truly it's not a dumb question. I know I've got kids (3-4) and they can TEAR UP any caulking laid down in a matter of months. For clarification, I am talking 'caulk', the sealant to keep the puddles of water from water-logging and destroying walls/flooring/underpinning etc. in between the tile and bathtub;  grout is a different issue, of course (as others have stated, grout being the 'cement' between and under your tiles, holding them in place).

Back to the silicone, honestly, I have found myself caulking their entire bathroom down only to find it 8-10 months later peeling back/cracking/ missing places... It truly depends on what is going on in said room how often one might wish to reapply/replace. I use a "holy hell, how can this be possible?" sliding scale to determine the NEED for replacement- however, as stated, with the kids and appreciation for puddling up the entire bathroom floor; I make sure there are NO breaks in the seal where water might seep through and cause damage... I'd say a good rule of thumb is if you feel like water (or any amount of moisture) could collect/ make it's way through- it's probably a good idea to knock it out. If it peels/flakes away easy, is receding from either side, starting to crack or chip, etc. there's a good possibility that it can/will allow water damage under- causing MUCH BIGGER issues... 
   Much rather do it and not need it than need it and not do it! haha