A lot depends on how original the game is.
(Sorry, but I'm going to be a back patter.) I've worked on my game for ten years. The game I have now is so incredibly different than the one I started with. A few core (and imaginative) designs remain, but as a whole, it is completely different. I've playtested for years with two different groups, and probably run over 70 convention games. The feedback was vital. Without it, there is no way the game would be as good. But, that's just the start....
Now, a designer needs to think of marketing. What sells, what doesn't? How does their game sell? Are there pieces, more than one PDF, books, etc.? Who can produce the product? How is the product going to be layed out; is everything in one giant book or are there several?
Then, if you're creating your own world, it needs to be understood by any and all who read it. This includes the adventures too. A monumental task, just as difficult as creating a logical and cohesive game design. I mean, all of us can create a city and run an adventure on probably three pages of material. But, if you're creating an RPG, others need to be able to do that with your three pages - hence, your three pages turns into fifty.
Also, art work, page layout, ugghhh EnDesign! For those not skilled, this is hundreds of hours of labor right here.
So, can someone do it. Yes. How long will it take? That depends on the depth of the game. If you want it to look and play anything like a "good modern RPG" then expect to put years into the creation process. (Which, by the way, is a complete blast! Wouldn't trade that time for anything.)