I could use CAD since I know how to, but I just use Photoshop.
Sometimes I don't even work with layers. Sometimes I do, mostly if there are lots of glass. Getting a glass surface I'm satisfied with is quite hard though.
Normally it takes half an hour to two hours to draw a simple building. I usually don't draw for longer, so complex buildings can take up to a week if I only draw for two hours a day.
I mostly do 1:1 from the start. I did a few drawings in other scales but got disappointed with the details everytime I rescaled.
Better do 1:1 from the start than to fix a rescaled drawing.
One trick to get a nice 1:1 drawing is to cheat when it comes to number of floors or columns of windows. Nobody's gonna count anyway and my drawing looks similar enough to the real building.