What's your investment threshold?

Cables are only part of the puzzle: room treatments, racks, footers, fuses, contact enhancers -- the list goes on.

For my system, cables are 22% of the cost of the entire system and other "tweaks" a further 29% i.e. over half of the cost of the system is in accessories 🤭

I use a Kimber KS1016 ($1,100) from my bought used $250 DAC to my ’tube buffer’ VAC Standard ($1,400 used)
So I am guilty as charged, of using a much more expensive cable than the electronics it is connected to.
On average though, my electronics have been in the $5,000 camp.

Years back I used to be in the 10% camp. That is 10% of the cost of equipment on cables.
If I include the power conditioners in the electronics and not the wire group, and my soon to arrive Magnepan 20.7 speakers.. I will be back in that 10% camp for total cost of IC.

I am resisting totally the power cord upgrade path beyond my Pangea powercords.
I used to be a non believer in powercords too. But bought a pile of Pangea years back and like them. I firmly resist any pressure to experiment further with powercords as a trap and a curse i have no interest in getting into. Ditto Duplex wall doohickeys.
I am totally at the final IC for my system for life.
I spent $7,000 on 5 pair of IC* and they will have to be it for good.
(4) pair of 1.5 meter Kimber KS1016, and one 7meter pair of Kimber KS1116. I still have a few Cardas Parsec in the system too.
(I have fancied a high end AES/EBU cord for a grand instead of the $39 one I now own... but that is only a fantasy so far.)

Speaker cables have been, and will remain Kimber 8TC.

I would say the 10% rule really is one to offer to newbies to have at least a starting point.
For everyone who is a savvy audiophile, the numbers can be ANYWHERE. From using the cheapest wires with a $100,000 system, to using the best wires with cheaper electronics. It can be anywhere. Since it is the sound/ end result, and not some 'theory' that is what matters.

I've never gone to the trouble of limiting, and therefore, quantifying cables as a percentage of system costs. I just buy what I think will work best. Doing a quick and dirty computation, cables come out to about 33% of total system costs. 

Add in the many cables I've boughten, it gets a lot worse, so I just factored in the contents of my present system. I don't regret it one bit.

All the best,
Nonoise

Do you impose on yourself a percentage range for cable spending relative to other components?

No. I don't crunch numbers, I listen to music.
That doesn't mean that there aren't cables that I can't afford, but I don't look at any system percentages when buying, only $$$ values.

I have spent $1,500 for speaker cables for a $6k amp.

Does $1,500 speaker cables count as compared to your amp, or your speakers, or both?

As far as cables go I pretty much always buy used and have learned there is probably less correlation to $$ and great sound then with other gear. Plenty of relatively speaking affordable cable can sound great in a suitable system. Also there are so many great deals on used cables its easy to buy and try and resell and try another. I have personally maxed out at $6-700 but for cables that retail anywhere  from $15-2500. And I have another one on the way, I must be sick or something :p