Talmud Berachot 12b says:
וְאָמַר רַבָּה בַּר חִינָּנָא סָבָא מִשְּׁמֵיהּ דְּרַב: כֹּל שֶׁאֶפְשָׁר לוֹ לְבַקֵּשׁ רַחֲמִים עַל חֲבֵירוֹ וְאֵינוֹ מְבַקֵּשׁ — נִקְרָא ״חוֹטֵא״. שֶׁנֶּאֱמַר: ״גַּם אָנֹכִי חָלִילָה לִּי מֵחֲטֹא לַה׳ מֵחֲדֹל לְהִתְפַּלֵּל בַּעַדְכֶם״.
And Rabba bar Ḥinnana Sava said in the name of Rav: Anyone who can ask for mercy on behalf of another, and does not ask is called a sinner, as it is stated following Samuel’s rebuke of the people: “As for me, far be it from me that I should transgress against the Lord in ceasing to pray for you, but I will teach you the good and the right way” (I Samuel 12:23). Had Samuel refrained from prayer, he would have committed a sin.
This seems to impose an obligation with no possible end. I know many people, and "the needs of your people are great" (Berachot 29b). Everyone needs prayer. So can't I always ask for mercy on behalf of another? Can I ever stop asking for mercy on behalf of those I know?
If I am aware my friend has a particular need (he is sick, or needs to get married, or wants children, or wants a better living, or wants his prayers answered or any of a gazillion other things), does praying for them just once fulfill my obligation? Once per day? Every tefilla?