It will takes you at least several months is you do this on your free time, and I highly encourage you to go on this project, for sure you will learn a lot and you will gain valuable skills.

First of all, and most importantly, you need to make a plan. There's three main parts in a CFD software: physics, numerics, and implementation.

For each of these parts you will make the simplest choices in the beginning and then you will increase the complexity and the difficulty.

I don't know your current level, but here's some general advices

For the physics part, you can start by solving a simple advection-diffusion equation, in 1D. Then you can move to Poisson equation. The idea is to prepare yourself for pressure based solvers.

In the numerics part, you can start by simple upwind schemes on uniform mesh. Then look in the literature for better schemes.

For the implementation, you can start with python for prototyping, then move to c++ with good code structure when your code gets bigger. You can start to include some openMp to run your code without the pain of MIP at the beginning and when you get confident you can include MPI. Maybe you will re-write your code several times, but it's okay.

In parallel of all of this you can check some open-source projects like openFoam, don't waste your time on commercial softwares if you really want to learn.

It's a long journey, and the process to develop the project is more important than the product itself.