I also think the length of "now" is dependent on the context used by the speakers, though I've encountered instances like:
Me: "Are you done with the text analysis? We have to turn that in next week"
Person A: "Not yet, I'm doing it now."
It could mean:
a. Person A is has been doing the thing at the moment, in which we can assume that he/she had started some time ago. But how long? Probably depends on the activity or how fast the person does it.
b. Person A is just about to start doing it, like saying "Now, let's start with our activity on yada yada yada yada" which means the guy is about to procrastinate and might be lying. In this case, 'now' is like a minute or a few minutes, or probably 'not existing'.
On the case of "Now, you know!", probably it starts from the time of realizing something and may continue on forever (unless it's a mistake and someone corrects you about it).
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