Am I gonna hear it if monitors use 2 diff length cables?

Brace yourselves for some “heressy ” :-)

Let me rephrase the OP’s questions two ways.

1. “will I hear a difference if…”
This we cannot answer because we don’t know your level of attention to detail nor critical listing acumen. You judge your perception personally.

2. “Will a material difference in the monitoring path create a detectable difference in perceived sound?”
In my decades at a mastering studio I can report YES. I have had engineers come into the shop complaining something’s wrong. Unknown to them there had been a seemingly irrelevant capacitor or cable temporarily swapped while a repair was underway and we forgot to tell them. They didn’t know what was going on but they detected the change. That’s because they operate in a highly calibrated monitoring environment every day, all day,for decades. They are acclimated and in a position to recognize minute changes.

Now if you don’t have that environment or years in place, probably will go unnoticed. Will it sabotage a mix? Maybe, most likely not. Level of anxiety should be low.

But…

It’s not right on its face. And it’s not hard to put equal lengths and quality of cable to your two speakers. Easy. Remove seemingly irrelevant errors wherever you spot them. Dot all the i’s and cross those t’s. Good engineering is mostly about removing small errors tjat are cheap or free. More about avoiding slowly boiling the frog, rather than being super creative
/thinking outside the box.

Brace yourselves for some “heressy ” :-)

Let me rephrase the OP’s questions two ways.

1. “will I hear a difference if…”
This we cannot answer because we don’t know your level of attention to detail nor critical listing acumen. You judge your perception personally.

2. “Will a material difference in the monitoring path create a detectable difference in perceived sound?”
In my decades at a mastering studio I can report YES. I have had engineers come into the shop complaining something’s wrong. Unknown to them there had been a seemingly irrelevant capacitor or cable temporarily swapped while a repair was underway and we forgot to tell them. They didn’t know what was going on but they detected the change. That’s because they operate in a highly calibrated monitoring environment every day, all day,for decades. They are acclimated and in a position to recognize minute changes.

Now if you don’t have that environment or years in place, probably will go unnoticed. Will it sabotage a mix? Most likely not. Level of anxiety should be low.

But…

It’s not right on its face. And it’s not hard to put equal lengths and quality of cable to your two speakers. Easy. Remove seemingly irrelevant errors wherever you spot them. Dot all the i’s and cross those t’s. Good engineering is mostly about removing small errors tjat are cheap or free. More about avoiding slowly boiling the frog, rather than being super creative
/thinking outside the box.

On the other hand, the speed of light is wicked fast.

The awesome power of ridicule has reformed me.

I shall repent my heresy;

Use a coat hanger on the left and a light bulb filament on the right.

No diff.

Perfect.

Only the song matters.

If you add one drop of water to the ocean (excluding all other mitigating factors) the global sea level will indeed rise.

The question is can you detect it?

There will always be someone out there who claims they can but for most of us mere mortals… 🤨

Umm if I actually have to be serious for once, a copper cable has a latency of like 5 microseconds per 100m. You’re not going to use 100m from an interface to some speakers, even if you did it would be completely imperceptible.

I’m all up for fringe debates but this is like saying my mate smashed down a reinforced concrete building with a sponge hammer. It’s a good anecdote but that whole pesky science and math comes into play at some point. Some things can’t really be rebuked because without said knowledge you wouldn’t be able to y’know build stuff.

This isn’t the whole I created 40 dimensions to solve a mathematical hypothesis sorta thing.

I mean I'm all for pedantry and shit, but then the idea that there's even such a thing as 'matching cable lengths' (let alone, matched frequency response for drivers, or a perfectly symmetrical room, or whatever) kinda goes out the window and renders the whole debate moot.

And let's not pretend like we're good enough recordists or mixers to claim that these sorts of nuances would have any real-world impact on our work when we're still trying to sort out tenths of a db of snare eq (on a coarse analog pot, no less). Put bluntly, we're gonna fuck up way bigger things on the average tracking session than a fraction of a microsecond's worth of comb filtering...

Ha, yes! Great anecdote.

Thinking about all the times (yes, plural...many, many timeS) I've sat there painstakingly tweaking an EQ to find the PERFECT setting between a couple options a half db away from each other, certain I'm hearing the difference, only to realize the damn thing was in bypass the whole time. Oops.

Pretty sure whatever difference comes from an extra four feet of cable is gonna be lost on a dolt like me 🤣