hmm...30 seconds stopped being enough for me after the first go round. I know kojic acid can lighten the skin but I mainly use it as a daily exfoliator. There are harsher acids, which I use for chemical peels, where its instructed to stay on your skin for several minutes. I would say those acids aggravates melanocytes much more and would quickly make the skin darker, when exposed to sunlight, if it wasn't followed up by adaquate moisturizing and, eventually, skin lightening products.