If building a cable I’ve already worked out the design/build for, effectively zero.
If working up a new design/geometry, or exploring new connector options/materials, maybe a few feet of raw wire over the whole prototyping process. Sheathing is more wasteful, long term, since it’s invariably much faster to assemble a cable by using a couple of extra inches on each end, for each run, every time you build and using that as a feeder to facilitate routing it into connectors or through splitters.
Initially it was finding a raw wire that I was happy with, but since then it is is “winding” each run of cable from the multiple raw conductors, spacers and binders.
Each cable in the prior post (there are two, one per channel) is comprised of multiple individual raw wire conductors, each of which is individually insulated, and then wound around a central core, using a combination of spacers and binders, and a retaining wrap.
The geometry of my cables is complicated (dual, interleaved, precisely spaced and bound, helixes) and takes up to an hour per mono-foot to do properly. Contrast that with being able to do a pretty, tight, 8-wire braid, as you see in most commercial third-party cables, at a rate of about a foot every 5-10 minutes.
Running the high-density plated copper-matrix shield over that takes about a minute a foot, and the sheathing about the same.
For headphone cables, there’s no real trigger … I always build my own. Haven’t found anything that sounds better, I have unique features and an electrically driven design that no one else does, I can vary the aesthetic to suit my whims, but above all … the modular design is important to me from an efficiency/flexibility perspective, given the number of headphones and amps/sources I need to accommodate.
Interconnects and AC cables are another matter, and some of those are commercial, some I build myself.