You know when your loo looks a bit grubby under the water, and you chuck a bit of bleach in? Does the bleach just de-colourise the grime and leave colourless grime coating the porcelain, or does it actually break the grime down so that it gets flushed away?
What HSMM said. I only tend to bleach my loo once a month or if there's been a d&v bug in the house. I've seen people recommended cheap dishwasher tabs for limescale removal in toilets (just drop it in & leave it to soak).