My favorite one out of the four turtle-drawings too. I canīt really remember how long it took. All the turtle images were sketched out at the same time and I was trying to keep them quick to get some energy into them. This one was the quickest out of all of them tho, while Raph was probably the one that took the most time since I used that one to nail down the style I wanted for them all.
Sketch...? SKETCH!? You call this a SKETCH!? There are no stray lines, nothings transparent, there's motion blur and it almost looks like a photo! I'd say this went way past the level of "sketch" a long time ago.