A difficult question, but understandable if you see the lot of detail.
I use oil paints and every layer needs a week to dry. My paintings have about 5 or 6 layers. I work from the background to the foreground (of course), what means the background mostly only has 2 or 3 layers and in the end the subject itself is built up of 5 or 6 layers.
I work on a several paintings simultaneously. This means I work with a rotation schedule and some paintings might take months from start to finish.
Apart from the technical aspects, the drying also forces me to stand back and reflect on a painting in progress. After working in close contact on a canvas for a while, it’s always refreshing to see it in another light, in another room and from a distance.
So, it’s hard to say how long it takes to finish one single painting. I never have tallied the hours and I do all kind of other artistic things in between, but on average I finish 8 paintings a year.
After finishing and signing, oil paint needs to dry thoroughly for at least 6 months, or better a year. Only then it’s ready to get the final varnish. So, if you see a painting made in the year 2018, it is not earlier sellable than 2019.
Here is a Youtube link to a bunch of short ‘making ofs’ of my paintings (and a few other projects).