There is something of a bounded limit on how long a single operation could possibly take, assuming your hardware isn't just plain faulty.

Yes, for the purposes of Big O, we define operations such that they aren't simply 1:1 instructions to the hardware, but they should be basic commands offered by your programming language.

If the basic commands in your language are taking extremely high bounded amounts of time, that's a sign your programming language is extremely poorly optimized, not that Big O isn't useful.