I read the book in a month - 1 chapter per day (about an hour per day). I found it engaging, though long-winded in spots. I didn't have as much of a foundation in liberty then as I do now, and I didn't have trouble with it - it actually might help if you're unfamiliar with some of the concepts that she espouses in the novel, rather than reading things you already know and getting bored with it quickly.
I'm not sure how well-read you are on economics, but I'd suggest Hazlitt's "Economics in One Lesson" and Rothbard's "What Has Government Done To Our Money?" if you're a beginner and want a primer on the basics, before jumping into Atlas Shrugged.
I'm currently working on Ayn Rand's collection of essays on capitalism (non-fiction), and find them to be very powerful arguments for capitalism with many in-depth examples. (FWIW)