Istighfar is a constant need of every believer, because all humans sin and 'the best of those who sin are those who repent,' and Allah loves the ones who turn to Him. The Prophet ﷺ, though sinless, said, 'O people, repent to Allah, for I repent to Him a hundred times a day.' Allah connects istighfar to relief and provision, quoting Nuh: 'Seek forgiveness of your Lord... He will send rain upon you in abundance, and give you increase in wealth and children' (Quran 71:10–12). There is no fixed limit, so a Muslim should make it a regular habit, especially after prayers, before sleep, and in the last third of the night.
Evidence — the daleel
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Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.