This hobby is for my spirit. Laying down a hard rule puts a shadow over the whole endeavor, so I won't do that. I will, however, judge the cost/benefit for a component (and cables to me are components) too high and decide it's not worth it. Once I cross that personal threshold, I start to regret spending the money and feel like I've been taken, neither of which is a desirable outcome.
However, through this process I've come to learn that for me spending 100% of an electronic component's value is where the benefits really taper off. For example, I own an Arcam CDS27 CD player. The dealer who sold it to me insisted I have a "modest" system and therefore I should buy modest cables. He literally made threats in a joking way and also touted his decades of experience. However, I borrowed the cables he recommended and they didn't work for my ears.
I finally ended up with a Nordost Tyr 2 RCA interconnect on the Arcam CD player. By trading in my old Heimdall, I was able to get into a 0.6 meter cable for $1,575, or about 100% of the cost of the player, and it sounds fantastic: natural, open, relaxed but also dynamic when it needs to be. Exactly what I wanted. If I'd settled on a lesser cable, the whole system never would have gotten "there" for me, so essentially it would have been wasted money. If I don't like how a system sounds or it's fatiguing, I tend to not even turn it on.
I'm currently running the Tyr 2 interconnect with Kimber 8TC cables that I bought as a stop-gap because I was spending a lot of money at once, and while they are bit muddy, I like the tonal balance. I tried Heimdall 2 speaker cables and found they accented high frequencies too much and gave a kind of punchy, night club type of bass--that combination wore on me through extended listening sessions. So, I'm thinking of buying Kimber Monocle XL speaker cables and if that doesn't get me "there," looking for a used pair of Nordost Tyr 2 cables down the road.
I know that with Nordost Heimdall 2 and Frey 2 make the most sense for my "modest" system, but it's really about what gets you there. As long as you can afford it.