Terminating unused wiring in electrical box
Cap off hot and neutral

The hot(s) and neutral wires should be capped off and laid in the panel somewhere out of the way.   Don't attach even the neutral; neutral can be a live wire in certain edge conditions.  You can attach ground, however.  

How long to leave the wires? Long enough so every hot and neutral can reach every breaker space in the panel.  This should be standard operating procedure for every circuit anyway*.  Neutral is one of these because AFCI/GFCI. 

* which is incompatible with the "Captain Snippy" method of making wires minimum length for an "ultra-neat panel" -- but you don't actually get brownie points for that.  You get brownie points for a functional panel.