I think all airlines have a 1 year validity, so you have a year in which to use your return ticket. So if you are wanting to stay longer than a year then youre best off with 2 singles..
If you will be coming back within a year then you have to book a "dummy" return flight when you book. This is usually about 6 months into the trip and can be changed free of charge. it sounds silly but you have to do this as they dont release details about flights that far in advance so when the time is coming up to your "dummy" departure just ring up the airline and get them to book your real return flight
hope this makes sense