Most round trip tickets have a maximum stay of 12 months.
TRAVEL FROM LAST STOPOVER MUST COMMENCE NO LATER THAN
12 MONTHS AFTER DEPARTURE FROM FARE ORIGIN.
And tickets are generally valid for one year from date of purchase, not travel date:
ITINERARIES MAY BE REBOOKED UP TO ONE YEAR FROM
ORIGINAL TICKET ISSUE DATE
Might make sense to buy a cheap round-trip for the outbound with the expectation that the return would be thrown away. Then purchase a return trip as needed.