[On the Obligation of Seeking Forgiveness]

Answer: In the answer there is detail:
3- The one who commits a major sin such as associating partners with Allah, fornication, and so on—this type must seek forgiveness and repent to Allah.
4- The one who is negligent in leaving obligations, and falling short in them, and wasting something of them—repentance and seeking forgiveness are obligatory upon him.
5- The believing person who leaves disobedience and fulfills obligations is not free of shortcoming and heedlessness; therefore seeking forgiveness is obligatory upon him.
So, the obligation of seeking forgiveness upon the first type is the most emphasized, then the second type, then the third type.
As for the time when seeking forgiveness becomes obligatory: it is when its cause occurs; and its cause is committing disobedience or leaving an obligation. So whenever something of that occurs, seeking forgiveness becomes obligatory.
As for the third type, the time when seeking forgiveness becomes obligatory upon him is the time when he senses and feels shortcoming in obeying Allah, thanking Him, and remembering Him.
And it is not hidden that it is from the nature of the believer to feel that, even if he reaches the utmost in obeying Allah, thanking Him, and remembering Him. For the believer remains accusing himself of shortcoming in obeying Allah, thanking Him, and remembering Him. And from here the Commander of the Faithful, ‘Alī (Peace be Upon Him), said: “The believer does not reach evening nor morning except that his soul is, with him, accused”—meaning: suspected.
And further: seeking forgiveness is among the completions of faith and its consequences. Do you not see how Allah—Exalted is He—described the faith of the Messenger and the believers at the end of Surah Al-Baqarah in His—Exalted is He—saying: "The Messenger has believed in what was revealed to him from his Lord, and [so have] the believers …" until His saying: "And they say, 'We hear and we obey. Our forgiveness, our Lord, and to You is the return'" [Al-Baqarah:285].
And finally, we say: seeking forgiveness has no fixed limit, but it appears to me that it becomes obligatory, when one feels sin and shortcoming, as a constrained obligation.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3