I teach my students to say 'and' only after the word 'hundred', although it also appears in many speakers for the sake of rhythm if there are a number of consecutive zeros-- the number of zeros per se is not important, however: 40,500 = forty thousand, five hundred 40,501 = forty thousand, five hundred and one 40,551 = forty thousand, five hundred and fifty-one 40,001 = forty thousand (and) one
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