With no written agreement if he says "We agreed for full repayment within 36 months" and if you respond, "I never agreed to a date", the court has to choose which account is more credible and, even assuming no formal agreement, whether a prior pattern of payments establishes a schedule or what term of repayment would be reasonable.
For example, if you borrowed $12,000 and made monthly payments for the first 24 months, such that repayment would have been complete in 36 months, then stopped paying, that history makes it credible that the debt was to be repaid over 36 months.
"I can't afford to repay my debts" is not actually a defense to a debt. If you are in terrible financial shape with no reasonable way out, you should consider consulting a bankruptcy lawyer.

