Not OP, and this thread is three weeks old, but I would estimate somewhere around 20-40 hours for your first build. I've been... very slowly... working on my 2.4 for just about the past two months. But most days I don't work on it at all and when I do I'm lucky to get 1.5 hours of time.
I've basically completed assembly, other than attaching the panels. On the to-do list are tensioning belts, setting up my klipper, and then tuning.
If you have the time, and an all metal hot end, you can also print your parts on basically any printer. I printed mine on a reasonably stock i3 clone.
Sorry for the delayed answer, I was on vacation with little digital exposure.
It is difficult to tell how long it took and what you all count towards the effort, but for me certainly more than 20-40 hours.
Just tuning the old printer and printing all the pieces to spec took several hours.
Then researching the options, learning and understanding how to build and how it works.
Also quite a bit of work was to research and source all the pieces of the BOM (I am in Europe, so I had to order from at least 5-6 different shops.)
This all before you even start to build....
Then the build itself and troubleshooting any issues that come up. Big shout-out to an amazing community here that helped me out once or twice very quickly and patiently.
Not to discourage you or anybody else, I enjoyed doing all of it, not only the build. It is also very rewarding doing everything yourself and in turn intimately knowing your machine.
And then the whole upgrading comes into play and it starts over again in smaller doses ;-)
I would wholeheartedly recommend to give it a try, just don't underestimate the commitment!