I tried a few weekends ago. (3rd evening perf and 4th matinee). I was front of the queue on the Saturday (I started queueing as the matinee hit the interval) and stayed till 8pm. They had no tickets and the box office was crazy busy because they had over 100 people turn up with fake tickets. So didn't get a ticket and ran around the block to see Spring Awakening at the last minute.
On Sunday i joined a friend in the queue at 11ish. We were second in the queue. When the box office opened the girl in front of us got a ticket straight away. At about 1pm 2 premium seats got returned and we passed on those (too much money) and some guys behind us in the queue snapped them up. Then at 1:30 the box office called us up to the window as 2 row k centre stalls had been returned. $167 each and we snapped those up!
The box office people and security guys were all lovely and helpful. They said there had only been a couple of shows over the last few months that there had been no returns for. We later found out that 2 people behind us got tickets too.
So I reckon I queued for about 8 hours over the 2 days. Totally worth it but get there early!