if your input data doesn't scale then either

① you're writing a real-time control program such as a motor controller and you care about not just the asymptotic complexity of your algorithms but their worst-case execution time, in microseconds; or

② you should just do the computation by hand with pencil and paper instead of writing and debugging a program to do it for you

at the point that the input data becomes too big for ② to be an appealing option, n is at least 100, which means you probably care a lot about whether your program is O(n) or O(n⁴), at least if you wrote it in something slow like python

maybe the time to start worrying about it is after you start the program debugged and running for the first time

It can be useful in general, but it is hard to provide the type of tolerances expressed in physical units you’d expect in an engineered solution when all the constants are ignored.

I'm not sure what you mean. Sounds a bit like you might be misunderstanding what were talking about.

It is a bit rude to become confused and then assume the other person was the one with the misunderstanding.

I'm just curious about what you're talking about. Tolerances and physical units are not usually mentioned in relation to time/space complexity. I'm open to the idea that you may know something I don't, feel free to enlighten me.