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It only takes a few days to carefully construct one if you are more technically minded and careful. However, it can take ages and a lot of tinkering to calibrate it correctly. Remember that no two are likely to be identical. I certainly altered and adapted things to improve it for me.
If you've got plenty of hair be ready to pull it out in frustration...
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I have a 16 year old cousin who has made his own RepRap. He's pretty bright for his age, but from what he has told me, anyone can do it if they can follow directions. It takes some time, but it's not hard labor :)