Zakat due on loans differs according to the state of the borrower. If his is not able to pay; then the lender has to pay it just for one year. But in case he can repay it, the lender has to pay zakat for all the period. 

Zakat due on loans

If the borrower is facing hardship and cannot repay the debt, the lender pays zakat for the loan once it is repaid. If it had been with the borrower for several years, the lender pays its zakat for one year only. On the other hand, if the loan is given to someone who has money and is able to repay it, its zakat is due on the lender every year, because financial instruments being with someone able to repay them is like its being with the lender.[1]