May Allah forgive your mistakes and guide you to the right path, Ameen!
Estimate how many days of Salah were missing. You may estimate the number of days every month after you became an adult (when monthly menstruation starts) when praying Salah was obligatory. However, the days of menstruation you don't have to pray. Therefore, a sister should exclude days of menses for every month during those years of missing prayers. That will bring the count further down.
To repeat missing Salah, you will pray Fard Salah for that day (2+4+4+3+4 and 3 rakat of Witr). That is, 20 rakat for one day of past when it was due and you did not pray.
To start making up for past years Qadha, two ways are recommended:
1. After each Fard Salah, pray one Qadha Salah.
2. Pray Qadha Salah whenever you have time - instead of any nawafil and Tahajjud . You can pray Qadha Salah at any time which is not Makrooh (around sunrise, sunset and Zawal time). For example, you may pray Qadha Salah during late morning, afternoons, nights.
One day of missing prayer is 20 rakat from Fajr to Esha (including Witr)
2 + 4 + 4 + 3 + 4 + 3
If you allocate 1 hour daily, you can make up for 2-3 days of the past.
Start today with some prayers and beg Allah for steadfastness.
And Allah knows best.
Ask Alimah Team
Note: The rulings giving herein are based on the religious rulings of the Islamic Law and do not have any implications on the Law of the Country. The rulings given hereunder are specifically based on the question posed and should be read in conjunction with the question. AskAlimah.com site bears no responsibility to any party who may or may not act on this answer. AskAlimah.com site being hereby exempted from loss or damage howsoever caused. This answers may not be used as evidence in any Court of Law without prior written consent of the web site.