How does one make up for prayers and fasting that were neglected for many years, then one repented, knowing that one might not be able to make up for all that was missed? And what is the ruling on this according to the four madhhabs?

The Muslim nation has agreed on the obligation of prayer for every sane, adult Muslim. It is the greatest pillar of Islam after the two testimonies of faith. Whoever denies its obligation is an apostate disbeliever. As for one who abandons it out of laziness, the majority of imams hold that he should be asked to repent, and if he does not, he should be killed. The Hanbalis consider him a disbeliever. Therefore, whoever has abandoned prayer out of negligence must renew his faith, offer sincere repentance, and perform many supererogatory acts of worship and obedience. He is not required to make up the prayers or fasts he missed, unless he wishes to be cautious and avoid scholarly dispute. As for one who only abandoned fasting, he must make up the fasts he missed.