Oil paintings can take 30 mins or several years. Take a little wander with me as I look back at my process for Smoke Break.
This is my reference photo which I took. It's my (then boyfriend) husband's hand.
1. Foundation
I like to paint in a highly realistic style so I need the best foundation possible. It is perfectly alright to paint an oil painting without a drawing, but that's not the way I roll. I find that I have less issues later on if I know for certain my foundation is correct.
In this drawing process I use a ruler to graph my points. For instance, I want to know where the edge of the watch is so:
I take my 8x10" reference photo and I note that the edge of the watch is 7" in and 9.5" down.
Then I have to do some math to find that point on my 20x24" panel. (7/8)*20=17.5" and (9.5/10)*24=22.8
Therefore the edge of my watch must be at 17.5x22.8" on my painting
... it's tedious and probably overkill and definitely my least favorite part
2. A little more foundation
I spray my drawing with a fixative so it doesn't smear in my paint
Then I mix oil paint with mineral spirits and put down some thin layers.
This base layer will give a baseline for tone. It's difficult to start an oil painting on a completely white surface. It's harder to cover the white and you lose context for tones.
3. 1st layer... 2nd layer...3rd later...
I will often do 2-3 complete opaque layers. It helps me to master value, it helps to define the lines and shapes correctly and the way light flows through layers of paint makes it more vivid to our eyes.
Certain elements will get a 1-2 more layers of glazing. Glazing is when you mix your paint with a lot of oil "water it down" so to speak to alter the color and tone. When you glaze a part of your painting it will always darken. So if I know I'm going to glaze something I try to make it lighter than I want it to end up.
Conclusion
I don't often paint in 8 hour chunks. I prefer to work in 3-4 hour chunks. So a painting like this will often take me a month(ish). But all in all it's anywhere from 80 to 100 hours.
So if I worked 8 hours a day 5 days a week on it. It would take me: 2-3 weeks.
Oil also has a longer drying time. On a large painting it's not that big of an issue for me because if one area is wet and I want to wait till it is dry I can just work on another part of the painting. It gets trickier as you get closer to the end.
You also want the surface to be completely dry when you glaze.
**the final step is varnishing the painting. The varnish keeps it clean and makes the surface a consistent shine. It's recommended to wait a year before varnishing an oil painting. But most working artists (myself included) will varnish after the painting is very dry (a few weeks).