Whoever deliberately abandons the Prayer for a Long Period, how should He make up for it?
Question :
Regarding the matter of making up for prayers, is it permissible for a person to make up for what he has missed, if he has not prayed for a number of years? And should he make up for it by praying double prayers, or what should he do?
Answer:
When a person abandons prayer for a number of years, then repents and guards his prayers strictly, it is not necessary for him to make up for the prayers he has missed. If this were a condition, it would discourage many people from repenting. The one who repents is ordered only to adhere strictly to his prayers in future and to perform many supererogatory prayers, acts of charity and good deeds. And Allah accepts the repentance of His servants.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 389-390