Tempo and Measure are both relative terms, they are not necessarily measurable. Time and distance are measurable. Tempo and Measure serve to allow us to be specific enough to convey concepts and ideas, without being lost in the minutiae of the tenth of a second. This is important, because time and distance change to rapidly in a fight to cover every possibility.
Measure is the relative distance between two opponents. There are only 3 measures, Out of Measure, In/Wide Measure and Close/Narrow. The actual distance these measures can be change because of things like weapon and arm length. You are out of measure, when you need more then one step to reach your opponent. You are In/Wide Measure when you are one step away from your opponent, but cannot reach your opponent with your hands (ie come to grappling range). Finally, Close/Narrow measure is when you can reach your opponent with your hands.
So, why doesn't the actual distance matter? It does matter of course. However, in a fight you don't have time to recognize that you're 3 feet away from someone with a knife and you have a sword. You can recognize that you're in measure, while you're opponent is out of measure. This shortens the mental "arithmetic" that you do, increasing your reaction time.
So, when facing off with an opponent take their measure. Note not only weapon length, but arm length, and height. All of these will affect when you're Out of Measure, In Measure or close measure.
A single tempo is the amount of time it takes to complete one movement. This movement can be anything, a cut, a thrust or a step, and we don't care if it takes 1/10th or 1/2 of a second. Some people are faster then others, and so they may have a slightly faster tempo then another. However they can only do one action at a time, and that's the important thing. You can't make two different cuts at the same time, nor can you easily redirect from one cut to another. Even empty handed it's one punch at time, or both doing the same thing at the same time. So, it doesn't matter if it takes 1/10th of a second to do something, it only matters that you can do one thing each tempo.
In general once you're stopped motion and begun moving in another direction, you've started a new tempo. The exception to this would be a chamber and release. If you're moving your weapon into it's "loaded" position ( hand cocked back, sword withdrawn for a thrust etc), then it can be a single tempo for the "cock, and fire" sequence. This doesn't mean that you aren't telegraphing a movement however, even if it doesn't take an extra tempo.
This leads us to 3 possible points at which we can act, during our opponents tempo. We can act before, during or after. While you may have just gone "duh", it's important to note that all three of these reactions can have different results. If you act before, you can miss your cover, or give you opponent a chance to interrupt your tempo.
Tempo and Measure interact, they are not solitary concepts. A long measure takes a longer tempo to cross. If someone is faster then you, extend the measure to slow them down. If you're faster then your opponent, close the measure to speed up. Learn to control both the Tempo and the Measure, and you control the fight.