It's a common question and one for which there is no simple answer.
Many people walk away from their "regular" job in the web field. Some make it, some turn back and a few turn to the side and find something new altogether.
If you're ready mentally (that's the first step and can take weeks, months, even years to really be prepared) it becomes a matter of figuring out the logistical ability to make the switch. The more you are "tied down" to assets or dependants, the harder a shift is, the less commitments you have, the easier to float. Either way you should leave prepared for a slow start; even an initial client list can quickly turn from golden to hollow, and is rarely enough to cover the standard of living you had a week before.
Look at your cost of living today and your income. Factor in benefits like health care, paid time off, self employment taxes, even the seemingly small items like free lunches or travel perks, those all go away.
Total up your hard costs, the fun things you won't live without and that's your annual cost of living. Divide by 2080 for an annual 40 hour / week job. This is your initial hourly rate. Now you need to reduce to allow for billing, organizing, prospecting, pitching, and all of the other overhead time you will incur (plus any actual expenses). This is your effective must earn rate.
Compare your must earn rate to what you make now and go back to that mental place... are you willing to step life down in the interim? Do you have the clients lined up to get you to that minimum? How can you up things to go from a minimum to more than you make now? What benefits to living do you get by making the move? What do you lose (day #10 you won't be getting approved for a car loan, think about that right now)?
You should ideally have 6 months saved up to float. I like more, some people like less, but 6 is reasonable... a few months to try, a few months to cover finding some work if you don't make it as far as you want.
Take this all together and decide; does the change give you enough to support the plunge and if so, is there enough work lined up or coming to make it possible.