Pagecount is less representative than you might think, simply because one page might have just a single image, whereas another might be a five-minute flash animation, and another still might be about 30K of text. It's hard to judge on raw page numbers.

I'm pretty sure that Rose's entry to the Medium is towards the end of Act 2. Act 5 so far has covered almost as much content (I think) as the entirity of acts 1-3, so you've a way to go yet.

And just wait until you hit "[S] Descend" :)