Stretch your thumb and little finger apart - as a rule of thumb, that's the distance you want to have between your mouth and the microphone. But it depends on the mic, the pop filter quality and the effect you want to create - closer to the mic and you will get a richer sound but more plosives, more sybilants and more mouth noises (clicks, licking, swallowing, etc); further away and you will get more 'room noise' and a weaker sound but fewer plosives, sybilants and mouth noises.
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