How much extra wire should I have in my electrical panel?
Leave just enough "extra" wire to route the wire along the edge of the board so it looks clean like the image below. Otherwise, do not bother leaving extra wire. You can splice more wire within the panel if you need to move things around later. Leaving a lot of additional coiled wire to try to account for the future could result in excess heat build-up and potentially an electrical fire. Note: Some excess wire is harmless, but if the panel begins to look like a mess of coiled wires, then you are likely creating the following situations the NEC tries to protect against in several areas of code (though not necessarily specific to the panels) to make sure the wire will not exceed the heat rating of its insulation at the expected current load:


Current running through a wire generates heat and relies on air to carry that heat away from the conductor. Therefore, the NEC protects against too many current-carrying conductors in close proximity for other areas of wiring:


  310.15(B)(3)(a)  More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
  
  314.16 Number of Conductors in Outlet, Device, and
  Junction Boxes, and Conduit Bodies. Boxes and conduit
  bodies shall be of sufficient size to provide free space for
  all enclosed conductors. In no case shall the volume of the
  box, as calculated in 314.16(A), be less than the fill calculation
  as calculated in 314.16(B). The minimum volume for
  conduit bodies shall be as calculated in 314.16(C).

When wire is coiled or there are more than one conductor, there is potential to induce additional currents in the wires due to the additional electromagnetic fields not otherwise present with a single conductor. The NEC protects against this related to ferrous metal enclosures and raceways:


  300.20(A) Conductors Grouped Together. Where conductors carrying alternating current are installed in ferrous metal enclosures or ferrous metal raceways, they shall be arranged so as to avoid heating the surrounding ferrous metal by induction. To accomplish this, all phase conductors and, where used, the grounded conductor and all equipment
  grounding conductors shall be grouped together.
  
  Exception No. 1: Equipment grounding conductors for certain existing installations shall be permitted to be installed separate from their associated circuit conductors where run in accordance with the provisions of 250.130(C).
  
  Exception No. 2: A single conductor shall be permitted to be installed in a ferromagnetic enclosure and used for skineffect heating in accordance with the provisions of 426.42 and 427.47.




  
  Image obtained from https://en.wikipedia.org/wiki/File:EEUU%28GE%29BreakerpanelInnards.jpg