Rescheduling a return flight - what are the limits
I'd like to book a round trip ticket with a long stay but I'm not solid on the return date. Round trip quite a bit cheaper than one way, so just booking two one ways wouldn't make sense. But say I do book a round trip ticket and six months later I decide I want to stay longer before making the return trip, how far out can I move the return trip? And would I be able to cancel the return and get a credit?
Edit: Trip is LIS to PHX, depart on July 19, return (for now) is Dec 27.