She has made up the prayers that she missed when she was younger and wants to carry on making them up
Question

                My mother did not pray for several years when she was young, and when she heard from one of the shaykhs that it is obligatory to make up missed prayers, she regretted it and repented, and she promised Allaah that she would make up the prayers that she had missed so long as she lived. In fact she has fulfilled her promise and has made up for all the years during which she did not pray. But she says to us: “I have to keep on praying because I said when I made a promise to my Lord, ‘as long as I live’ and I am still alive.” She offers the obligatory prayers and not the naafil prayers, although she knows that she hads offered all the prayers that she missed. Is what she is doing correct? Is it regarded as a vow?.
Praise be to Allah.The majority 
    of fuqaha’ are of the view that the one who does not pray for a while after 
    he reaches puberty has to make up the prayers that he missed. If he does not 
    know how many prayers he missed, then he should do what he thinks is most 
    likely the number of prayers that he missed. 
It should be 
    noted that the prayers that she missed when she was young, before reaching 
    puberty, do not have to be made up, because she was not accountable at that 
    time. 
Some 
    scholars are of the view that whoever deliberately misses prayers does not 
    have to make them up; all he has to do is repent and do good deeds in the 
    future. 
See the 
    answers to questions no. 7969 and 
    72216.
Anyway, what 
    is required of your mother is to seek forgiveness a great deal, repent and 
    do naafil deeds in the hope that Allaah will accept her repentance. Allaah 
    says (interpretation of the meaning): 
“And 
    verily, I am indeed forgiving to him who repents, believes (in My Oneness, 
    and associates none in worship with Me) and does righteous good deeds, and 
    then remains constant in doing them (till his death)”
[Ta-Ha 20:82]
With regard 
    to her saying that she made a promise to Allaah to make up the missed 
    prayers for as long as she lives, this is a vow to make up the prayers that 
    she missed, and she has done that and fulfilled that vow. So she does not 
    have to continue making up prayers that she has in fact made up, because a 
    prayer can only be made up once. 
As for her 
    saying “as long as I am still alive”, it seems that what is meant by this 
    expression is that she will continue to fulfil her vow as long as she is 
    still alive, and she will never stop making up prayers because of sickness, 
    being busy or any other reason that may distract her from praying. 
If she wants 
    to carry on praying, this is a good deed that was encouraged by the 
    Messenger  (peace and blessings of Allaah be upon him) when he said: 
    “Prayer is the best thing prescribed, so whoever can offer a great deal of 
    it let him do so” (narrated by al-Tabaraani in Saheeh al-Jaami’ 
    (3870)) – so long as that is done with the intention of offering naafil 
    prayers, not with the intention of making up missed prayers. 
And Allaah knows best.