I've been at this since the late 80s and still have a day job.
My best friend who writes a lot more books (3-5 a year) and gets paid way more than I for her work, ALSO has a day job.
Yes, you can get to a place where you can support yourself with your writing, but you won't have health insurance and all those other handy, necessary perks. A regular paycheck every week keeps you fed and the roof from leaking.
My friend has had cancer and cannot get health insurance--not without paying a bomb for it--because of the pre-existing condition. Her spouse also needs specific health care. They're not old wrecks and falling apart, they're just average people who have to pay attention to things.
For myself--my day job is self-employment for small time internet sales, and business has fallen off.
I've got no insurance at all except for the house and car. If I get sick...it would be BAD.
Get a day job to bring in the cash and perks. Writing is a risky, uncertain business and doesn't pay well.
And yes, back in the day I thought all I had to do was sell ONE book and I'd be set for life. I'd heard all about those lovely, life-changing 6-figure deals.
My advance worked out to about 1100.00.
Show Me The Money--have a cold dash of reality for earnings in one of the more steady, high-paying genres.
My agent says only look at the lowest figures on that page. The high numbers reflect special circs.