As a matter of fact, with some external tools like MCEdit, it's not really as time consuming as it seems. A lot of large scale of Minecraft redstone contraptions are modular in nature, once you build and validate the basic idea, scaling up is just a matter of copy-pasting.
In my experience, the initial research/designing phase takes quite some hours (typically a good weekend), scaling up is must faster. In addition, most players who are into this typically have a number of creative worlds serve as libraries of past builds, so you'd almost never start from scratch.
In my experience, the initial research/designing phase takes quite some hours (typically a good weekend), scaling up is must faster. In addition, most players who are into this typically have a number of creative worlds serve as libraries of past builds, so you'd almost never start from scratch.