I have a Word study group starting up later this month, so I've got experience with this. i would say the average beginner user could prepare for the tests in 1-2 months overall (i.e. working around a full time job). This would be spending a few hours a week reading through the study guides and practicing in the software. Obviously it could be faster if you have more hours to devote to it or you are starting out with a higher baseline of experience.
My study group uses the official study guides to prepare. I would say anyone could also prepare by downloading the exam objectives and googling the various items.
I found the exams pretty straightforward, but I have been teaching classes for years, so that's not really relevant necessarily.