Is it obligatory for one who has forgotten a part of the Qur’an to re-memorize it, especially if their memorization was not firm, and if it is obligatory, what is the daily amount for memorization?

Forgetting the Quran due to neglect and dereliction is a sin for which one must repent, and one must review and memorize what has been forgotten. There is no specific amount for daily memorization; rather, a Muslim should exert effort to the best of their ability.