Sometimes you get inspired and it takes a couple of days, sometimes it takes weeks. This sort of stuff eats time like you would not believe. A good part of it depends on your abilities, knowledge, and skill, as to how deep into this sort of stuff you can go. That in turn determines how long it takes. For every image you have been shown in this thread are probably 4 or 5 I didn't save. I don't usually save those I am not happy with the outcome of.
Some of my early ones you can see a distinct lack of that knowledge displayed in the appearance of the image. You never learn it all. You can get better than you were but you will never reach the end of knowing it all. (or at least I won't) The complexity of each program is enormous. I may swap an image between two to five different graphics programs to get the desired effects. Sometimes they come out right, sometimes they don't. Sometimes just the lighting setup is enough to make or break an image. Do you use an environmental light like the sun as your main lighting, or do you use the 3 point standard photograph illumination? How wide is the light to be if you use a pin point light? Where should it start illumination and where should it end? Should it cast shadows? Should it be another color other than white? Should it be a pin point light, a light box, a spot light, or a battery of them? How strong should the light source be? Should you use gel filters or not? Where should you place them? All of this only deals with the light source to illuminate the scene. There's cameras, pose, the scene set up, just to name a few, and every one of them just as complicated as the lights, if not more.
Here is one I didn't do well. It's over lit, the posing is poor, the whole thing doesn't fool the eye at all. In my eyes it is a total failure beyond the learning experience.