Surprisingly, if you keep the feature set down, time to ship wasn't bad. The project was done in about half a semester and that's only working on it here and there. For the final project we had to implement three "advanced" drawing tools, I think one of the ones I came up with was to draw Bezier curves, now that wasn't exactly trivial and could definitely add some time to ship.
Depends on how feature complete you want it to be.
Back when I was working in Processing a lot, I could probably have whipped up something decent in an easy weekend. If I wanted to have more complicated features like flood fill, cutting/moving regions etc, I might need an extra day to figure it out.
If all you want is a bare minimum scribbling lines in a limited palette, that could probably be done in a matter of minutes.