"Full Fare" tickets are valid for 12 months from the first date of travel. If the return half if unused, as others have said, it depends a bit on the airline and even who you talk to but in principle you pay the difference between the fare you did pay and what the fare is now for the one way sector. Schedules are released about 330 days before departure. This is due to the various computer systems and the fact that may of them work on a purely ddmmm basis rather than ddmmmyy (or even ddmmmyyyy).