I've worked on a site that was ~100 million pages, and I've seen indexation percentages ranging from 8% to 95%. When dealing with sites this size, there are so, so many issues at play, and there are so few sites of this size that finding an average probably won't do you much good.

Rather than focusing on whether or not you have enough pages indexed based on averages, you should focus on two key questions: "do my sitemaps only include pages that would make great search engine entry pages" and "have I done everything possible to eliminate junk pages that are wasting crawl bandwidth."

Of course, making sure you don't have any duplicate content, thin content, or poor on-site optimization issues should also be a focus.

I guess what I'm trying to say is, I believe any site can have 100% of it's search entry worthy pages indexed, but sites of that size rarely have ALL of their pages indexed since sites that large often have a ton of pages that don't make great search results.