I believe the art was done in 4 weeks, though some may have been polished or added post-release (since it's up to version 1.11).
The writing was more like 1 week, IIRC. Not sure how long the translation took.
Music was creative commons (at least originally, might have been replaced), so who knows when that was done or how long it took.
So yeah, mostly done within 4 weeks. Sendo worked tirelessly on this one, and I think it really shows.