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From this: "if the electrical length of a trace is longer then 1/10 wavelength, you need to treat it as a transmission line."
The 1/10 wavelength applies to the speed of the signal on the PCB (about 6"/ns for FR4 board material), which is less than the speed of light .
For digital signals, from this: "The generally accepted standard for this boundary, often called the “critical length,” is when the propagation time for the signal to reach the end of the trace and return (the round trip length) is equal to the rise time."
Note that it's not the frequency of the digital signal that's important, only the rise (and fall) time.