I used to say half a day and half a lifetime ;-)
Seriously, it varies a lot, It varies whether the work is on paper or panel, whether it is multi layered or a few simple layers quick one completed in an afternoon. I am much slower using cold wax and my completion rate is way down . . months not days. However, in the old days, with en plein air, I tended to paint fast. But I do spend a lot of time gazing and thinking, before applying a stroke of a brush.