Personally, I make enough from shooting that, if I did quit (or lose) the day job, I could probably scrape by and survive on it alone. I'd have to stop buying gear, because then all my photo/video money would have to go to bills. Right now, I'm playing both sides of the fence, and I'm able to pour all the money I make shooting, back into more (and better quality) gear, with a little left over because I've sorta run out of things I want to buy - which then in turn allows me to charge higher rates and take on more ambitious gigs.
In the back of my mind, I always say "hey, if I can get this to a point where it could take over, I could quit my life-sucking day job and just do this full time!"... but I know me. The more likely reality is that I'll probably just keep doing both for as long as I can feasibly juggle the two... I've done 100% freelance for years at a time in the past, and it was very feast-and-famine - so inconsistent. The day job sorta sucks my life away, but the paychecks are good and they roll in like clockwork. I try to look at it like it's my most frequent client - boring as it may be. But I am also very happy to have photo/video as my slightly less lucrative "plan b" in case something happens.