Biz tickets booked in "I" class tend to have 6-month or less applicability; look for "D" class tickets if you want 12 months for the trip.

Note the "tend to have" - there's no cast in stone rule, but QR seems to follow what I said.

If the tool you use to search for fares doesn't let you specify the booking class, just make your return date over 185 days or so and you'll almost certainly be shown 12-month fares.