Well, it depends on your goal. If you want to make a "purely mechanical" pinball machine with limited artwork, you could probably make one of those in less than 6 months.
On the solid state side of the coin... I think your experience is extremely relevant. I'm assuming for this argument, that you are strictly a "one man show" and also have a full time job? If you have to start "learning it all" from day 1... one machine would be challenging, no matter what your time frame. I would investigate the different hardware and software possibilities and choose what makes sense for your project.
It took me a while to really understand and grasp what is happening with the "pinball software" side of pinball design. I know the most about the Mission Pinball Framework [MPF] at this point and it's Python based. If you have any experience with Python, that can really speed up the software side of that learning curve and save you some time as well.
Open Pinball Project, fastpinball.com, Trident Pinball and a few other resources might be some good starting points to help track down what resources you need as your "partners in crime". In fact, very recently, Trident is offering some crazy cool things like taking your drawing/design and actually cutting the playfield for your project for you.
Just because you are a one man show, DON'T FEEL LIKE YOU NEED TO "REINVENT THE WHEEL" UNLESS IT'S ACTUALLY REQUIRED FOR YOUR PROJECT. Plug here for the Yoppsicle LEDs. They will save you a LOT of time... https://www.cometpinball.com/products/yoppsicle-led-stick
MANY others have not only created homebrew machines, they have cleared a path and laid down a road for you to already follow. They have made 1,000 of mistakes ans assumptions that you don't need to also make. So, spend some time researching solutions and documentation of completed hamebrew projects and mimic the shortcuts and helps that others have found.
That will really give you a kick start in the right direction and help you decide what SOFTWARE and HARDWARE "clicks" for you and makes the most sense for your project.
Also, I should mention that if you have ever made a MAME cabinet or even a "virtual Pinball" table, that can help tremendously with some of the cosmetic aspects and DIY resources for "common part" things like playfield support, coin door access, cabinet design/hinges and other aspects of your machine/project.
Heed this: If you are not mechanically inclined, don't like to soldering, have no idea how to troubleshoot basic pinball problems like "why exactly does this not work", and you hate making wiring harnesses from scratch for hours until your fingers hurt, then a homebrew pinball machine is probably NOT the project for you.
