There is no set "regular" amount of time it takes. There are too many factors involved. Down here the brown stuff can last for several months, while in other areas it might be practically gone before you ever notice it. Excess silicate + phosphate = diatoms, and there's no way of knowing how much you have unless you test for them, and even if you knew it's still anybody's guess. Either one doesn't really tend to fuel diatoms all that much, which is why sometimes some people who use ordinary sand don't have problems, but the combination of the two is pure trouble. new tanks usually have too much of both, to a varying degree, but once either one is used up enough the diatoms crash. How long does it take? There's no telling. You can help speed things up by avoiding use of silica sand and by avoiding overfeeding. Skimming helps, and if you wipe the glass clean, be super-duper careful to remove the brown algae from the tank instead of just knocking back into the water. This will a: keep it from spreading and b: export these excess nutrients. Eventually the good coralline algae & other more desirable organisms will be able to use up all the fuel and the diatoms will vanish.
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