21. The rent calculated in accordance with section 20 has effect from the month following the month in which the application was filed and remains in force for a period determined by the lessor, which may be from 1 to 6 months without exceeding the renewal date of the lease.
Upon the expiry of that period, the former rent shall be re-established unless the lessee demonstrates that he may benefit from an extension of the reduction for another period.
Where the decrease in income is permanent, the rent reduction shall, notwithstanding the first paragraph, be granted for the remainder of the lease.