Q. Rashi (21: 18) quotes Talmud (Sanh. 71b) that the wayward and rebellious son is executed on account of what he will do in the future – he will stand at the crossroads and rob people, killing them, thereby incurring the death penalty. Baaley Mussar and others argue that if this applies to future wrongdoings it surely should hold true for mitzvos to be done in the future, as long as there is a certainty that they will be so done, (As opposed to Yishmael that was judged only “Baasher Hu Shom,” since the crimes to be committed were by his descendants). According to this opinion, how long does one have to be doing a mitzva so it is considered as a sure thing and he can be given already now the credit of the future? This can make a tremendous difference on the judgment of the Yemei Hadin.

A. Indeed we do find that in merits Hashem does consider the future good deeds to be done. Rashi (Shemos 3: 12) writes; Concerning what you asked, “what merit do the Bnei Yisroel have that they should go out of Mitzraim?” I have a great thing dependent on this exit, for at the end of three months from their exit from Mitzraim they are destined to receive the Torah on this mountain. Similarly, when Moshe Rabbenu was about to kill the Mitzri, Rashi quotes Medresh Rabba; He turned this way and that way, and he saw that there was no man: he saw there was no man destined to be descended from him [the Mitzri] who would become a ger.
In Taanis (8b) we find that Rabbi Zeira and his followers accepted a fasting day they could not in practice keep, due to a decree of religious persecution. He based his decision on (Daniel 10:12); “Then he said to me: Fear not, Daniel, for from the first day that you set your heart to understand, and to fast before your G-d, your words were already heard.” This verse indicates that from the moment one turns his heart to fast, his prayers are already heard.
Rabbenu Yonna (Shaarei Teshuva 2: 10) clearly states that from the time one accepts in his mind and ascertains decisively to follow the right way, he already acquires the merits of the mitzvos and their reward, He quotes the Mechilta (Bo 12: 28 – Rashi) “Once they accepted upon themselves [to do the korban Pesach], Scripture credits them for it as if they had [already] done so.
Horav Dovid Kronglass zt”l (Sichos Chochmo Umussar 3: 1) is one of the sponsors of the idea that future mitzvos can be counted on the Days of Judgment, and he maintained that thirty days would be the necessary time to create a chazaka of future continuity. We find the thirty day chazaka in many different instances such as establishing marital status (Rambam H. Issurei Biah 1: 20), establishing someone as a cohen, the correct name of a person or when in doubt what was said on Tefilos that change with the seasons (Shulchan Aruch O.H. 114), and other instances.
However, Horav Shmuel Kamenetsky Shlit’a told me that seven days may be sufficient.
Horav Shlomo Miller’s Shlit’a opinion is that the main factor that may grant a prerogative on future mitzvos not done yet, is the true commitment of the individual in the performance of those mitzvos and even just three days may create a chazaka. The Rov added that “Machava Tova or a good thought is counted as an act” (Kisdushin 40a), applies only when the good thought are really kept.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a