Revisions. The word sends a chill down any creator's spine.
Maybe it is because of my background in Engineering, but I'm used to customers requesting changes, and the stress they put on a project. In the art for hire world, it can be very similar. Art-work can be purchased as part of a larger project or as a one-time request, but regardless of whether deadlines are firm or loose, it is very hard to estimate the impact a requested change can have on your timeline.
Working on commissions one is usually facing a loose deadline (but it is in your benefit to make it a hard one. More on that later), but changes can mean a lot of work for an artist. It is up to the artist to ask for as much reference and details regarding the work in question-- vague descriptions and a lack of commitment on the commissioner's side can quickly turn into a revisions nightmare.
It is fair to assume there can be a number of changes when you're developing an image for someone else. But how many of those changes are reasonable, and how drastic can they be?
For example: A commissioner (let's call him Bob) gives you a couple badly drawn references of a D&D character he created. In the references, the character is an anime-style knight girl named Lucrezia with plain armour-bikini and a sword. Bob asks you to draw 'just any action pose' with her.
You sketch an action pose and show it to Bob. "Hmm, I don't know", Bob says. "Could you make it more 'action-y'?". This usually means the customer has no idea what he wants, but what you showed him is not it, and so you go back to the drawing board and sketch another different pose. "Oh that's better!", Bob chirps, "But could you turn Lucrezia's face to the viewer looking sexy?", despite the fact that in that pose it would be anatomically impossible for the character to turn her head toward the viewer, and so that means sketching another complete pose that makes sense. And this is assuming, for the sake of this thought experiment, that you and the customer have the same idea of what "sexy" means. In custom art made for fans, anatomical impossibilities are often overlooked for the sake of T&A.
You finally hit a pose that satisfies Bob and continue working on the image, giving Lucrezia the same kind of plain armour and sword as in the references. "Oh wait", Bob interrupts alarmed, "the armour should be golden and with wings" or "I want her to have triple nunchucks instead of the sword". You can see where this is going.
This can go on for any number of revisions. Depending on your skill and proficiency, this can quickly spiral into many extra hours of what amounts to design and development time for which you're not being paid for, if you agreed upon a price when your services were requested. Depending on your skills and rep, you may be able to get away with telling a customer that the price will be calculated at the end, but even most professional artists usually can not do that.
Therefore, it is critical for you to have what amounts to a "Terms of Service" type agreement that the customer needs to be aware of, at the very least. In my personal case, when the customer doesn't ask for a specific pose, I allow for two different pose ideas at most without charging extra, I specify that if there are no references there is a design fee, and I try to leave any detail work until the very end when the commissioner has seen the final pose and given the go-ahead to continue working on it. This may sound a little harsh, and naturally as an artist it is your duty to ask for all the information you need to work on something before you start. There is a balance to be stricken between charging by the line and letting a customer walk all over you asking for infinity changes.
Ultimately, it is not possible to 'quantify' how much work goes in a piece of art. Maybe you're doing it for a friend and you want to put in a little extra effort at no charge, and accommodate unreasonable revisions. Maybe you're drawing something you utterly despise and want to get it out of the way as soon as possible. More in this in a future post-- since I mentioned you may be drawing things you may not enjoy, I have to talk about what types of content you're willing to draw and how this is relevant to you as an artist.
Here are a few warmup doodles from Bongo.