Nev I was also a cadet from the days before industry experience.
I joined QF with about 250hrs. Minimum required for F/O training was 1250 aeronautical experience (not sure if that is still the case, no reason to think it has changed). So I had about 2000 hours of flying to do (as S/O hours are halved for aero exp). Took me around 3 years to do this. In that time I did miss an F/O slot during a time of rapid expansion when guys who'd been in the company 12 months were getting them.
These days with industry experience this would not be a problem for any cadets and they can take an f/o slot pretty much as soon as their seniority allows (which if they're willing to move to Perth would be pretty much straight away).
So long as you meet the minimum experience requirements then time to promotion is purely down to seniority, not previous experience.