He did not pay zakah for ten years and he does not know the amount on which he should pay zakah for those years
Question

My husband has never paid zakah. He began working in 2009, and his salary was between 2,500 and 3,000. He used to put his salary in the bank, then spend from it on furnishings for the apartment every now and then. He does not know the amount of cash for which one year passed, or whether it reached the nisaab [minimum threshold at which zakah becomes due] or not, but after that, after we got married, we gave zakah in Ramadan 2019, as we managed to save money from Ramadan 2018. What should we do about the previous years and how should we work it out, especially as he does not remember anything about his savings, or when his savings reached the nisaab, or how much he was spending and when? But what I know is that around the time of the marriage, which was around Ramadan 2017, he had approximately eighty thousand; I am not sure of the exact amount, but it is around that figure. I hope that you can advise us as to what we should do with regard to giving zakah for the years that were missed. Is it permissible to give it not all at once? Please note that his salary increased at the end of 2017. Is zakah to be based on the original amount of money that was deposited in the bank or on the total sum with interest received?
Praise be to Allah.Firstly: working out the nisaab [minimum threshold at which zakah becomes due] for paper money
The nisaab in the case of paper money is to be worked out on the lower of the nisaabs for gold and silver.
The nisaab for silver is 595 grams and the nisaab for gold is 85 grams.
This was mentioned in a statement issued by the Islamic Fiqh Council of the Muslim World League, and a statement issued by the Council of Senior Scholars in Saudi Arabia. It is the view favoured by the Permanent Committee, Shaykh Ibn Baaz (may Allah have mercy on him) and others.
Please see the answer to question no. 2795  .
So whoever owns the nisaab must give zakah, regardless of whether he is saving it to furnish an apartment, get married, go for Hajj, or anything else.
Secondly: zakah on money that was put in a riba-based bank is due on the original amount only
If he put the money in a riba-based bank, zakah is due on the original amount only, and no  zakah  is due on the interest, because it is evil, unlawful wealth, and Allah is good and only accepts that which is good. What he must do is withdraw the money from the riba-based bank, or at least put it in a current account.
With regard to the interest received, it should not be kept in the first place, and he must get rid of it.
Thirdly: failing to give zakah does not mean that it is waived with the passage of time
If someone fails to give zakah for several years, he must still give it, and zakah is not waived with the passage of time.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) was asked: There is someone who did not give zakah for four years – what must he do?
He replied: This person is sinning by delaying his zakah, because what the individual must do is give zakah as soon as it becomes due, and not delay it, because the basic principle is that obligatory actions must be done immediately. This person must repent to Allah (may He be glorified in exalted) from this sin, and he must hasten to give zakah for all the past years. None of that zakah is waived. And he must repent and hasten to give it so as not to increase his sin by any further delay.
End quote from Majmoo‘ Fataawa ash-Shaykh Ibn ‘Uthaymeen (18/295).
Fourthly:
What your husband must do is try to find out how much he had each year, then if it reaches the nisaab, he must pay zakah on it. If he is not sure about the amount, he should err on the side of caution. He can go to the bank and request copies of statements, and find out the balance for every year, after deducting any interest received, if there is any.
Fifthly:
zakah must be given immediately, and it is not permissible to not give it all at once, unless there is a valid excuse such as not having cash available, in which case he may wait until it is available.
Sixthly:
He must repent to Allah (may He be exalted) for not giving zakah for this length of time.
And Allah knows best.