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.

I was thinking mostly photoshop brushes, not paint tbh. Thanks for sharing. might try at home!

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.