I do not build sci-fi and I wonder: when do you decide the model is complete? (as this is not our logic)...
For building subjects actually known as existing subjects (like an NCC-1701 Enterprise or X-Wing), it's finished when the model looks as close as I want to the original (which may or may not be before, as or after the parts in the box run out!
).
For subjects that I have created from my own imagination, the build stops when the model meets what my imagination thinks it should look like.
Same as when I'm building aircraft or tank models. If it's a real thing, the model stops when it looks as much like the real thing as I want it to. If it's a made up aircraft or tank, it stops when it looks like my head thinks it should look.
Paul