The answer is the letter ‘f’.

1 year = 12 months (February = 1 `f`, comes only once in the spelling of all the months)

1 month = 4 weeks (first, fourth = 2 f’s, comes twice in a month)

1 week = 7 days (first, fourth, fifth = 4 f`s, comes 4 times in a week)

1 day = 24 hours (four, five, fourteen, fifteen, twenty-four= 6 f’s, comes 6 times in a day)

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