The outbound flight must be taken within 12 months of ticket issue. The inbound flight must then be taken within 12 months of the date of departure of the outbound flight.

As you've already flown outbound in June 06 (we'll say its 30th June 06, for the hell of it), your return flight must depart no later than 29th June 07.

This is all on the assumption you have a regular fare, not some deal ticket with other restrictions or complicated IATA fare.