Counting the time it took to build, use, and learn the limitations of my first layout, this one took two years.
With the benefit of learning what not to do, and what to do that I had not done, in all sincerity, it took me
an evening of doodling and a scale drawing of the final track-plan. I didn't really figure out bench dimensions and all that stuff. I measured as I went along with a basic plan to bolt four open grid modules together in a square and operate the layout from a central pit. So, with the basic layout in mind, for me the big task was to ensure the track plan was usable by both me and my then small stable of locomotives. Once I had curves, a yard, and grades figured out, plus the types of turnouts, the rest was just measure and cut, plane, sand, screw.....etc.
One tip: once you have a track plan figured out and are really quite keen on it, clear off the space it will occupy and lay 3/4" masking tape on the floor to simulate the track plan, particularly the main, sidings, spurs, and any yard or staging. Use green or blue painter's tape to map out the edges of any bench-work. Once you see it all laid out in 2-D using the tape, you'll get a better feel for your use of space, for curves, elevations, etc.
-Crandell