When I did this it took me a little over four hours to finish to my satisfaction. You don't have to put every little detail in. As a matter of fact, for this exercise, you probably want to simply leave out any type of shading and just focus on the lines.
And it is a lot to start drawing, but a little known fact about any type of art is that doing it well is going to take a lot of hard work. So, with that in mind, let me tell you a little about what this exercise is supposed to do for you. So I've blown up a drawing... am I getting closer to drawing what I want? Getting closer
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The Perceptual Grid. It's hard, but you can learn to see the world kind of like your original, gridded off drawing. What you see is your original, and what you're drawing is the one you're blowing up. Nifty, huh? Now, you're never going to get to the point where you literally will yourself to see lines when you're looking at something, but this exercise helps you become spatially aware; getting things in the right place and the right size on your paper. More on this a bit later