It all depends upon your question/research question to how long you can take some times the long term loans cannot be repaid or they are not in a position to clear the loan with interest accumulating to a higher level. It can be arbitrary but I suggest you to eyeball the period in your sample and accordingly take a median cut off to draw the line . Personally more longer the duration better the understanding about the loan amount and the duration of payment or not payment. Please do not consider the cases of Vijay Mallayya and Diamond merchant Neerav Modi for the amount of loan they have taken.
At the same time please look into the purpose of loan and the reason(s) for non payment, if any.