I giggled to myself a bit when I read this question. Even the Gods do not truly know how long a shot will take to complete. Of course, we estimate. After all, every project has a schedule and a limited amount of money. For a commercial or television project, the amount of time it takes to a create a shot will usually be more accurately in sync with the schedule as opposed to a high-budget feature film where there is more creative latitude and the budgets are substantially larger. The producer, animation supervisor, and coordinator will come up with a number to use as a guideline. On a feature, you might have 3-4 weeks for a 3-4 second shot with a couple of characters. When I worked at Meteor Studios in Montreal animating dinosaur shows for the Discovery Channel, we were lucky if we had a week to animate 5-6 characters, and this was with interactions like fighting or eating. Most shots I animated on Avatar took anywhere from 2-6 weeks. Sometimes you get lucky and hit a homerun, getting a shot approved after only a few days of real work. Ultimately, the factors affecting the schedule of a shot are varied, and the chain of command leading to its final approval can be long and, in some cases, very redundant.
There are many different opinions about what decides that a shot is actually finished. Some will argue that it's the director who has final say. Some might say it's the studio executives who most often throw a wrench into the works. Or some simply believe the shot is done when the money runs out. The fact is that all of these are true. Depending on the show, the company you are working for, the director, and the studio, any number of factors can determine how long it will take to complete a shot. It is also very important to clarify one thing: The amount of time you actually work on a shot is not equivalent to the amount of time the shot is on your plate. On a major feature film it isn't uncommon to have a shot sit on your plate for 6 months or even up to a year and a half. But it is wrong to say you've worked on the shot for that long, which is what you'll hear many animators complain about after they get notes on their 7th iteration 6 months after getting the shot. “I’ve been working on this shot for 6 months!”
In total, animation on a shot for a feature film could take anywhere from a couple of days to a couple of months at the extreme. The question is funny, simply because it is often nearly impossible to predict how long a shot will take to get approved. The short, seemingly easy shot can turn into an endless nightmare while the long shot filled with complex action could go very smoothly. The main ingredient that often sabotages the final approval of a shot is the “creativity factor.” Depending on how locked down the vision of the shot is, and more importantly, how willing the various powers that be intend to stay committed to that vision, can drastically alter the amount of time it takes to complete the shot. Storyboards, animatics and templates are there to help paint the vision of the shot before you ever work on it. If the director wants to commit to these tools, then you may see very little blocking changes and things can go smoothly. But if little time and commitment was put into them, then the floodgates of creativity open up and the artist can get caught making variation upon variation until either the money runs out and the work is forced through, or the director falls in love with one of the passes and it gets approved.
You see, the skill set of the animator is almost never the reason an animation takes a long time to complete. In fact, many animators will tell you they created 5, 10, even 15 iterations of an animation that went through a series of reviews lasting weeks or months, until eventually the blocking was approved and the animator could actually get into the business of polishing their work. And even after that, it is still possible to get directorial or editorial notes that require a return to blocking. In the wise words of one of my supervisors, “All of the versions you did were good. They just weren't right.” Edits can change, a director can be unsure of what he wants, your animation director has certain needs, and ultimately the artist is stuck in the middle trying to please everyone up the chain one step at a time.
So how much time does it take to create a shot? Roughly 3 weeks give or take a few months...depending...