There are roughly 300 million people in the USA right now. Let's assume that 99% of the US population is dead and transformed into undead walkers. That still leaves roughly 3 million human survivors scattered across the USA in small groups of 15-20 people -- not unlike Rick and his group hiding out at the prison. Math-guy ran some more numbers and figured out that there could be as many as 150,000 camps spread throughout the country.
Now let's assume that each human survivor dispatches an average of 10 walkers every month -- keeping in mind that some of these survivors are kids, old folk, and otherwise defenseless individuals. But let's assume that it averages out that way. That means that roughly 200 walkers on average are being killed by each survivor group every month. That means that roughly 2,400 walkers are being killed each year by human survivors. 2,400 destroyed walkers multiplied y 150,000 groups equals 360 million dead walkers.
Now given that walkers aren't having sex and creating baby walkers, the numbers indicate that all of the walkers should be wiped out by sheer attrition within a year. Additionally, both the comic book and the television series are pretty much in the second year of each incarnation, so it is unlikely that the walker plague should still exist -- at least not to the degree that we're seeing in this current season!
Of course, I already mentioned that there are some survivors who are incapable of successfully fighting back against the walkers without the help of others. Also, there are areas of the country where walker populations are going to exceed the ability of survivors to fight back -- I'm thinking of major cities such as New York and Chicago and Miami, etc...
But there is another factor that would appear to work in the favor of human survivors -- especially here in the USA. There are tons of weapons and bullets in this country. I can see groups and portions of the government hording those weapons, but I would imagine that we would eventually establish sniper bases to successfully reduce walker populations.
And, of course, there is the constant rotting that can't be too helpful for the walkers!