the instructions for Form 656 state:
Periodic Payment: This option requires you to make the first payment with the
offer and the remaining balance paid in monthly payments within 6 to 24 months,
in accordance with your proposed offer terms.
There doesn't seem to be any other option except 24 months. But are you needing an Offer in Compromise OR an installment agreement? An IA can go out 72 months. that is a Form 9465
