סימן יט סעיף ח
אִם נִסְתַּפֵּק לוֹ אִם אָמַר מַשִּׁיב הָרוּחַ אוֹ לֹא אָמַר, אִם הוּא לְאַחַר ל' יוֹם שֶׁכְּבָר הִתְפַּלֵּל תִּשְׁעִים פְּעָמִים כָּרָאוּי, חֶזְקָתוֹ שֶׁגַּם עַתָּה הִתְפַּלֵּל כְּהֶרְגֵּל שֶׁלּוֹ כָּרָאוּי. אֲבָל בְּתוֹךְ שְׁלֹשִׁים יוֹם צָרִיךְ לַחְזֹר וּלְהִתְפַּלֵּל. וְכֵן בְּטַל וּמָטָר אִם מִסְתַּפֵּק לוֹ לְאַחַר שֶׁכְּבָר הִתְפַּלֵּל תִּשְׁעִים תְּפִלּוֹת כָּרָאוּי מוֹקְמֵינָן אֲחַזַּקְתֵיה שֶׁגַּם עַתָּה הִתְפַּלֵּל כָּרָאוּי, וְקֹדֶם לָכֵן צָרִיךְ לַחְזֹר וּלְהִתְפַּלֵּל. (קי"ד)
If you are in doubt [about] whether you added mashiv haruach or not, [then] if thirty days have passed, [meaning:] you already said Shemoneh Esrei ninety times properly,9There is some confusion about the author’s use of 30 days and 90 Shemoneh Esreis interchangeably. Concerning mashiv haruach; 90 Shemoneh Esreis will be reached in 29 days because of the additional Musaf on Shabbos, and Shabbos comes at least four times in every 30 day period.
Likewise it has to take 34 days or 35 to have tal umatar said 90 times because we omit the middle 13 berachos on Shabbos and Shabbos comes either 4 or 5 times in every 30 day period.
The Mishnah Berurah (114:37) holds that we can rely on either number leniently. Thus, after 90 Shemoneh Esreis or 30 days, whichever comes first, we can assume that tal umatar and mashiv haruach have been established by habit. it can be assumed that now, too, you prayed [and added mashiv haruach] by virtue of habit. But [if the doubt arose] within [or before] thirty days [elapsed], you are required to repeat Shemoneh Esrei. The same [rule applies] to tal umatar. If the doubt arose after you already prayed ninety Shemoneh Esreis properly, we presume that now, too, you prayed properly. But before that (before 30 days), you must repeat the Shemoneh Esrei.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)