What is the ruling on studying the matters of religion that are known by necessity once again for one who has forgotten them, and is it permissible for him to abandon the Sunnahs and some of the obligatory acts during the study if illness has slowed his comprehension and increased his forgetfulness? And what is the minimum daily limit for studying this knowledge?

It is impossible to conceive of the existence of what the questioner mentioned, for what is known by necessity in religion are the matters and rulings of religion widespread among Muslims, such as the pillars of Islam and faith, and the prohibition of fornication and alcohol. This does not require full dedication. As for the obligatory knowledge, it is that by which a person corrects their creed, worship, and profession, such as knowledge of the pillars of faith, and awareness of the rulings of purification, prayer, and fasting, and what is lawful and unlawful in their profession. These sciences can be imagined to conflict with supererogatory acts of worship, and seeking knowledge takes precedence over them. What the questioner mentioned about studying taking a long time is not correct, for the Prophet, peace and blessings be upon him, taught the one who prayed incorrectly what would rectify his worship in a few minutes, and it is inconceivable for this to conflict with performing individual obligations.