Even though for some reason an answer was accepted within 17 hours, when most people would have waited for a few days to a week to hear from as many different people as possible, I will tell you what I know.
I work with freelancers who do graphics who depend on their freelance income and unless they got secret trust funds, its their only income. As with all other businesses, a good half fail within the first year. A lot more fail to last more than a few years. In my opinion, the skill level of the designer is not the benchmark to go by were you to bet who will still freelance full time in 12 months or who will not.
Freelancing is a business, and must be treated as such. You must have a capitalistic outlook that each one of your hours IS actually worth money, be able to easily say no when clients ask for freebies that make no business sense to give, be able to clearly separate business from personal spheres, be able to quickly evaluate how long a given task will take and what the maximum price that can can be set that gives the client value yet makes it worth your while. Freelancers must be able to manage time, and spend lots of that time marketing themselves and getting 'out there', and most of all they have to be very very organized.
You ever seen a big street intersection with three gas stations on three corners and a fourth gas station being built on the fourth corner? As a kid, I always wondered what kind of an idiot would invest in a fourth gas station when they could have built it a bit up the block. Turns out, as I learned in grad school, that being the 4th gas station on the same intersection is more profitable than building one down the block, though it seems to go against the grains of common sense.
So my advice, when there is no work to occupy your time, is to spend that time marketing yourself which includes participating in every single crowd sourcing 'competition' that you can find, and there are more than a couple of those daily. Today, graphics people who are competent using CSS/HTML/jQuery look sexier than their competitors (Photoshop/illustrator can no longer be your only tools if you are looking to improve your chances with freelancing).
This guy http://www.flyelanddesigns.com/ makes his living freelancing as a graphics designer, who is successful and made freelancing viable enough to live comfortably and support a family. So is freelancing viable? Of course, but far from guaranteed income.