How much time is required to prepare 3D animation video?

It depends on the kind of software you use, and the length of the video, a detailed animation video can take anywhere between 4 - 8 hours on an average. Nowadays, we have automated solutions which rely on AI alogrithims to create detailed scenes and short animation videos good for promotions and ads, although you have to pay them upfront, you can take a free trial with the ones you feel are good for your project.

One has to learn the basics of 3D animation and then practice to make impressive and attractive animations by using the various techniques. If you know how to make an animation then only the question comes as how much time will it take to make a particular animation video.
There are many short courses on 3D animations and one can learn the art of 3D animation through them. One can search for them in sites like Udemy, YouTube, LinkedIn, Classcentral, Coursera, Arena-Multimedia etc. At the same time there are many applications and working platforms in which one can produce one's own animations. Please note that learning and creating value animations is a patient process and takes time. Generally for making a few minutes animation video it could take time from 4 to 10 days depending upon the complexity of the design and the plot of the animation. There are many aspects that the animation creator has to consider and only after that a good product will be visible. The objects, their colour, their speed, the background, the music in the background, and many other factors are crucial in making a good animation. All this requires learning and practicing of the subtle art of animation.
Some of the platforms where one can practice and learn animation are Autodesk Maya, Daz3D, Blender, Cinema 4D etc and many more are there. Most of these tools come with a limited capability free version also.
What I would advise is first go through the basics by going through the videos and free courses provided in the internet in various related sites and find out that to what extent you want to learn this subject and then choose an appropriate course. A good course will be somewhere 6 to 9 months duration. After that comes the phase of hard work, good efforts, and practice and one can then comfortably create the videos of the professional levels. There is no substitute for learning and practicing. Whichever area of work you choose hard work and sustained efforts are the key to success.

What are the skills you have and what is your qualification? Eventhough you are new to 3D animation, if you have some related soft skills, it may become easy for you to make the video. But if you don't have any skills in areas belonging to this, it may take more time.
There are many courses that may help you in creating a 3D animation Video.
1. Advanced program in 3D animation. This course is offered by MAAC. Their website is https://www.maacindia.com/. This course will give you inputs that are required pre-production, production and post-production phases. They have similar courses and you can see their website and choose the best option for you.
2. 3D animation courses by Udemy: Their website is https://www.udemy.com/. They offer online courses. You can learn 3D animation software like Blender, may and unity on this site.
3. Domestika. It is also a good platform to get some good knowledge about 3D animation, The website is https://www.domestika.org/. They have many courses and you can select the best one that suits you.
4. CG spectrum is another place where you can learn this subject online. https://www.cgspectrum.com/ is their website and they have a number of courses and you will have good options there.
Once you learn the basic knowledge about 3D animation, you yourselves can decide on the time it is going to take for you.

There could be always time variation from man to man depending upon the expertise of the individuals. However, it could take you a couple of days to be familiar with the different techniques such as crowd running, cable size etc which could make you familiar with the multiple characters.
There are some studios producing 2 minutes animations in a shorter timeframe but for others, it could take even 20 days for making an episode of 22 minutes.
3D animation films having the amount of realism in the animation would take years for its completion because that requires a lot of attention in the areas of background, layouts, props and character designing.
Hence the bottom line is for a skilled worker it could take 2 to 3 minutes for 3 D animation bu for the complex animation, it might take months altogether.