Typically, most people do it in about 10 months, although many take several years.
The loop averages from 5,500 miles to about 6,800 miles depending on which route your take. Given the above milage, you do the math.
As to how to break up your trip by year, consider that most people do the route by season for obvious reasons. On the attached map you want to be on the Gulf of Mexico (purple) in the winter. Head up or be on the Atlantic Coast during springtime (green). Spend the summer on the northern leg (Brown) and be off the Great Lake and heading down the Mississippi River in the fall. Once the northerns start coming down after Labor Day, the Great Lakes can get rough. That leaves you heading back down to the Gulf Coast in the fall and on the Gulf for winter.
This is just the recommended game plan. It takes you out of Hurricane season on the Atlantic coast in the springtime, gets you through the Great Lakes before the weather turns rough, and puts you back down on the Gulf Coast well out of it's hurricane season. So, a lot has to do with your schedule and your starting point.
The Missisippi River is kind of boring up north with not much to see other than mud flats. What I will do when the time comes, is to go down the Mississippi about 1/3 of the way. When I hit Kentucky, I will get off the Missisippi and go down through Kentucky Lake and work my way down through the Tom Bigbee Waterway and eventually to the Gulf coast coming out of Mobile Bay, Al. It is a much more scenic route