I don't post much in here but I felt compelled to respond to this.
I've been doing IM full time since I was laid off from a 150k job in August, with a one month old daughter. Before that I'd been doing PPC campaigns for about 18 months, constantly wondering how much I needed to be making before I pulled the trigger and quit. I had months where I was "thank God I have this job to fall back on" and months where it cost me money showing up at a day job.
I had money saved when I got laid off, and my fiance has a good job (although she was on maternity leave for six months, not getting paid), but all of my campaigns had died and my main traffic source had died - so I basically started from scratch when I got laid off.
It probably helped that my last day job was the most retarded company I've ever worked for (wasn't my choice to work there, we got sold to them), and it was almost a relief. But instead of panicking, it was a revelation. Either I'm good at IM or I'm not, so let's find out. I did know that after fifteen years at corporate gigs, I was tired of sitting in a car all day to show up at a specific time just so some asshole can pretend he's a good boss. I live two blocks from the ocean, but sitting in an office and working so that maybe the boss of my boss of my boss of my boss might decide that I get a 2% raise at the end of the year. Nobody does any actual work at corporate gigs, it's just a crapshoot.
I'm not bitter, and I definitely miss showing up hungover at 11 on Fridays, surfing the internet for two hours, and getting paid no matter what. I don't have time management issues (other than not taking enough breaks) - it's amazing how little on the internet actual matters when your time is money. I also make better choices with affiliate networks and offers - I don't have the patience to spend two days building campaigns for the latest hot triple-brokered offer that ends up getting dropped three days later. I look at everything long term. There's one specific network to which I sent 100k in revenue the first six months of this year that I won't run with anymore, because I can't count on them to give me the support I need as a legitimate businessman.
If you won't give me an affiliate manager who can give me answers to my specific questions, then fuck you I'll take my business somewhere else.
If you can't tell me why my campaign was declined, fuck you I'll run it somewhere else.
It's amazing how much more insight I find in my data now that I'm not trying to cram work into a seven hour window after I'm done with a day job and my fiance has gone to bed.
It's scary at times and really stressful - but what worth having isn't? You can always go get another day job. I wake up two hours earlier than I used to because I can't wait. I get to have lunch by the ocean, spend zero on gas, take a phone call or meeting whenever I want, see my daughter whenever I want. I don't treat this as "I hope this works out"; I treat it like there's no other option because it's the only one I want.
My long-winded point is that I can't say for sure I ever would have reached a point where I felt comfortable pulling the trigger on a day job - part of that was that a day job was limiting my ability to expand the business, but also I know I got comfortable at times. It seems scary to quit your job, but when you take a step back, every great entrepreneur has none it, and usually with a lot more risk than we have. We sit at computers and set up campaigns that launch pretty quickly. You can spend a couple hundred bucks and determine if a campaign is going to make any money. We know the traffic better than most ad networks know their own traffic (I've worked at a bunch - they're retards).
People act like a day job is the 'less risky' option - that's a fucking joke. At some point at any company, your number is going to be called. Companies work for shareholders and nobody else. That's awesome as a shareholder. It doesn't matter how hard you work a corporate gig, how long you've been there; it comes down to numbers. As least when you work for yourself you have some control over your success.
Only you know what makes you comfortable. But just work hard.