kopite01
Back to the original question, so you can sleep better again: It’s 60 days in advance for that particular ticket type (ZVV regional fare network. single journey 3 zones)
However a single journey ticket for that approx. 10 minutes trip is only valid for 1 hour, not the entire day. So should your flight back to Straya be delayed by several hours, you might end up with a useless ticket you’ve ‘secured’ in vain weeks in advance (unless loving to hang around at airports).
As already stated above, such regional fare network tickets will still be available (in unlimited numbers) up to minutes before the train departure at exactly the same price. The passenger numbers in a train are NOT limited by ticket sales: The ticket is not a ‘booking’ for a specific train, it’s just an ‘admission fee’ which would also allow taking one or two later trains in your case (= during the 1 hour validity period).