Your "testing" will not really help you.

If you get transfer errors, you will know that it doesn't work.

But if you do not see any transfer errors, you will not know what safety margin you have. Maybe a 10C temperature change will be enough to break the communication. Maybe a small change in the supply voltages. Maybe a mains cable close by...

As Andy notes, you really have to read the datasheets and let them tell you how long cable you may have. And the datasheets has to suggest that the cable can be way longer than you actually need, so that you have a bit of extra safety margin.

What would happen if an elevator manufacturer just took a random wire for the elevator and then tested that the wire didn't break with 8 people in the elevator? Would you trust that elevator? Don't you think they first have to compute that wire they need? Then decide what safety margin they need? Then figure if they need extra margin for ageing and wear? And then - first then - starts with testing to verify that their computed values matches the reality (and that the wire manufacturer did not lie about the wire qualities)?

Never try to skip steps to try and save time. There is no time to save - every step later in the development process where you find an error will cost you much more time and money for corerction.