Delaying Making up Fasts: What Should You Do?
Question

One year I did not fast the days when I had my monthly period, and I have not been able to fast them until now. Many years have gone by and I want to make up the fasts that I owe, but I do not know how many days I have to make up. What should I do?
Praise be to Allah.In case of delaying making up missed fasts , you have to do three things: 

You have to repent to Allah for this delay, and regret your past negligence; you must also resolve not to do such a thing again, because Allah says (interpretation of the meaning): 

“And all of you beg Allah to forgive you all, O believers, that you may be successful.” [al-Nur 24:31]
Delaying making up missed fasts is a sin, so it is essential to repent to Allah from that. 

You must hasten to fast the number of days you think you missed, and (remember that) Allah does not burden any person beyond his scope. However many days you think you missed, you have to fast that number of days. So if you think that it was ten days, then fast for ten days; if you think that it was more or less than that, fast as many days as you think, because Allah says (interpretation of the meaning): 

“Allah burdens not a person beyond his scope.” [al-Baqarah 2:286] 
“So keep your duty to Allah and fear Him as much as you can.” [al-Taghabun 64:16]

Feed one poor person for each day if you can afford to do that, even if you give it all to one poor person. But if you are poor and cannot feed another person, then you do not have to do anything apart from fasting and repenting. 

What is meant by feeding a poor person here is giving half a sa’ of the local staple food for each day; half a sa’ is equivalent to one and a half kilograms. 
And Allah knows best.