MCU Design, maximum wire length per frequency and characteristic
There really is no general formula that's going to work in all cases. For starters, the upper bound on propagation speed is going to be the speed of light. However, the speed at which a signal propagates down a wire depends on the dielectric (insulation) around the wire. For a bare wire, this will be air and signals will travel at close to light speed. For traces on a circuit board, signals will travel at around half of light speed. Next, you need to know if the signal is actually propagating as an electromagnetic wave or if you're RC limited. This will depend on the length of the wire, the resistance of the wire, the distance from the ground plane or other conductors, etc. And then you have to look at reflections. If your wire is not a transmission line with matched impedance on at least one end, you're going to get reflections and you're going to have to wait several propagation delays for the reflections to die down before you'll be able to reliably recover the signal level.  And that doesn't even consider crosstalk/coupling from adjacent signals.