Most references cite about 10% of messages partially (or sometimes completely) decrypted prior to December 1, 1941, at which time a new version of the system went into effect, requiring the cryptanalysts to start again.
mˈoʊst ɹˈɛfɹənsᵻz sˈaɪt ɐbˌaʊt tˈɛn pɚsˈɛnt ʌv mˈɛsɪdʒᵻz pˈɑːɹʃəli ɔːɹ sˈʌmtaɪmz kəmplˈiːtli dᵻkɹˈɪptᵻd pɹˈaɪɚ tə dᵻsˈɛmbɚ wˈʌn, nˈaɪntiːnhˈʌndɹɪd fˈoːɹɾi wˈʌn, æt wˌɪtʃ tˈaɪm ɐ nˈuː vˈɜːʒən ʌvðə sˈɪstəm wɛnt ˌɪntʊ ɪfˈɛkt, ɹᵻkwˈaɪɚɹɪŋ ðə kɹɪptˈænɐlˌɪsts tə stˈɑːɹt ɐɡˈɛn.