There's no real particular percentage really because you really can pick up your game after entering the course.
It's also quite difficult to allocate a fixed % in a Maths exam because the difficulty of them tends to vary.
However, if I HAD to give some numbers, they would be around (with respect to actual HSC standard):
2U: Above 80%.
3U: Above 70%.
I've seen and taught people entering Year 12 with those marks (roughly) and then acquiring E4 in Extension 2. There are exceptions of course, such as students doing poorly in Year 11 and then suddenly doing well in Year 12, but the numbers above would be what I consider somewhat ideal.