-How long it would take me if I worked on it every Monday but took off every other day
-How long it would take me if I worked on it every day
It’s impossible to give you a concrete answer to these questions. How long it takes to animate something depends on a lot of factors, including your experience level, the type of animation your doing, the amount of detail in the animation, the tools you are using, etc.
The way I would approach these questions is to break down the work you need to do. You’ve already got a storyboard which is a good start. Start by counting up the number of shots in your storyboard. Then take one shot that you think is representative of the “average shot” in your project in terms of length, complexity, etc. and go through and animate it start to finish. Time yourself doing this, and then use that as an estimate for how long every shot will take, and with some simple math, how long your entire project will take in person-hours. Divide the total by how much time you want to spend on it each day, and you get a rough estimate of the number of days of work it will take you. It’s best to consider this an underestimate because things always take longer than you think.
And also, would I be able to do such a project in Pencil2D?
Yes definitely, but there is a smart way of using Pencil2D to do this, and a very bad way of using it. Don’t do it as a single project. Break your animation up into shots, and make a project for animating each shot. You can then export each project individually, and combine the clips together in a video editor. This is very important because if something happens to your project file (ex. it gets corrupted or deleted), you don’t lose several minutes of animation. By the same logic, also make backups of all your project files at regular intervals by copying the file to a separate folder or using File > Save As… and saving to a new name. These are good practices not just for Pencil2D, but for any animation (or really any program period).
Best of luck with your short film!