As an author my goal is that writing is my day job.
Very much likewise, as an end goal.
The most reliable sources I've seen converge on the genre SF/fantasy market in North America supporting about forty people making enough to not need a day job at all, though. And that's including the people at the Stephen King end of the scale. So aiming for it and doing everything feasible to attain it does not well translate to expecting it to happen.
But I suppose that lots of people have to pull down two jobs to make ends meat. So under that theory 8 hours straight job, 8 hours writing job?
A day job that could be relied upon to not expand beyond those bounds would be nice, I suppose.