Scyther's and Scizor's movesets are mostly the same except for these levels below. Scyther's move is first, Scizors second.
1: Vacuum Wave or Bullet Punch
21: Wing Attack or Metal Claw
37: Double Team or Iron Defense
41: Air Slash or Iron Head
First, note that once you have evolved to Scizor you can learn any of Scizor's level up moves from the move relearner with a Heart Scale. (You have to do this for the level 1 moves.)
So of the 4 moves for Scyther, probably the only ones you would want are Wing Attack and maybe Vacuum Wave, which both take advantage of the Technician ability. So I would say any time after level 21 would be fine!