What is the advice that must be followed by someone who used to pray intermittently (one or two obligatory prayers a day) then repented, and wishes to make up for missed prayers, the exact number of which she does not know, and fears that death may overtake her before completing them?

Praise be to Allah who has favored you with repentance from this grave sin. And know that scholars differ regarding the obligation of making up prayers intentionally missed. The more cautious of the two opinions is that of the majority, which is the necessity of making up those prayers that were intentionally missed. If you do not know the number of prayers you missed, then you should make an earnest effort to estimate them, making up prayers until you are certain or have a strong presumption that your obligation has been fulfilled. You must make up these prayers immediately, in a way that does not harm your body or your livelihood. It is not sufficient for you to make up one day's prayers with each (subsequent) day; rather, you must diligently make up what is due from you, unless doing so would cause you harm. If you strive to make up these missed prayers to the best of your ability, then you have done what is required of you. And if death overtakes you while you are engaged in making them up, there will be no sin upon you, for you have done what you were commanded to do, as commanded, according to the words of Allah Almighty: (So fear Allah as much as you are able).