I would say either level 34 when it gets fire blast, or level 43 when it gets Crunch.
If you have a good amount of TM's, I would say evolve it whenever, since you should be able to teach it moves like Flamethrower, Shadow claw, Grass knot or Solarbeam.
So I would say...
If your pansear has higher special attack, level 34.
If your pansear has higher attack, level 43
OR if you have a bunch of TM's, evolve it whenever.
Hope I helped!