How Long Is Too Long?

I've been applying for lots of jobs recently and I've filled in a number of application forms, which has got me thinking, how long is too long for an application form? I've filled in some which were only three or four pages, and some which are as many as ten pages long. Now this is hugely time consuming, not to mention that the vast majority of the information simply involves you repeating what's on your CV. One form I've filled in had seven pages of questions, many of which required a full page of writing. This took ages to fill in.
Now I'm not saying that application forms should be scrapped, but I don't think they need to be pages and pages long. I spent hours working on my CV to get it up to scratch, so it's very annoying for employers to refuse to even look at it, to insist that I rewrite the entire thing on a different piece of paper. Alongside relevant questions about your suitablility for the role, they should also look at your CV, to get a clear picture of what people's skills are and the experience they have.