Actually 1 hour, before my face started to hurt and my arms got tired. This thing is actually about 10 feet across in real space, and Nightmare Moon is about as tall as I am.
Tilt Brush is a virtual reality 3D drawing thingie. More like sculpting than drawing. Try searching for "tilt brush" on youtube to see videos of people using it.
Making something that actually looks like it's supposed to is a lot harder than it looks. Going to try practicing by doing a 1 hour speedpaint of a scene from each pony episode per day. S1E2 tomorrow.
