I don't remember exactly but it was by far the most involved legendary. I had to do so much planning that I made a spreadsheet! I needed to figure out what the player animations all did for every race, sex, and class, then determine what shapes I could make the sword turn into that would work for all of those shared animations.
I'm making this up for the sake of an example - I don't remember who does what off the top of my head: human female guardian might spin around for skill 2 clockwise while others spin around counter clockwise. This kind of thing meant that I couldn't have the sword turn into a one sided shape, or some of the races would be hitting things with the blunt side of the shape. So I needed to do a lot of planning to make sure everything would work for all 16 of the shapes.
I also had to figure out shapes that would work well for the classes that shared the animation. I didn't have the luxury of making class-unique shapes - it had to be based on the player animations, and multiple classes use the same animation.
In addition to that, I had to actually make 16 shapes, which was unprecedented. I was effectively tasking myself with making a whole weapon set just for one weapon - I didn't have time for that level of work. What I ended up doing was making some new pieces and reusing a lot of other pieces, including pieces of the original sword, to make unique shapes. This saved me a lot of time, but it was still a ton of work.
The base sword itself also took a lot of time. Also an fx artist had to do a lot of the effects.
So to try to give you an actual answer, best I can say is in the realm of a few months.