There are lots of choices for guitar grade exams. Classical guitarist can sit ABRSM, Trinity and LCM exams. Electric/plectrum guitarists can sit Trinity, LCM and Rockschool exams. There is also a preliminary or initial grade many of these boards offer.
There is a separate syllabus and rules & regulations book for each. Some you can pick up for free in local music stores, others you can order from the exam boards website.
ABRSM also do group exams called 'music medals' for classical guitar.
There is usually a local representative for each board who you can call up and have a nice wee chat with, explain your new to this, and they will help you

You'll find their number on the boards website.
Books-wise: Trintiy and LCM (and rockschool) publish exam repertoire in grade-specific books, often with supporting test information. ABRSM don't publish music like this so you will have to buy all the music separatly (can be a pain if you don't know the music!) There are CDs for each board you can buy with all the repertoire on it. This helps.
There is no such thing as an 'average student' and no average time to attain a grade. Each student will learn at their own pace and it will take them however long it takes them! I usually only put them in for exams when they are ready or very near-ready, they especially need to be secure if it's their first exam. There are supporting tests for each grade (aural skills, sight reading etc).
If your not sure how the grade system works, the best way to get to know it is to take an exam! Even if its grade one (the cheapest one ) or of course the closest grade to your level (probably quite expensive but worthwhile), it is a great way to understand not just the playing requirements, but how the process works and how a student will feel during the exam.
Good luck!!!