An Essay on Entropy: what is it, and why is it so important?

[ReneZ]

Do you have an estimation of how long of a text is needed to get decent entropy stats?

This depends.
For H1, which is generally not too interesting, already 1000 characters give an indication.
For H2, which is the quantity most looked at for the Voynich MS, at least 10,000 would be needed.
It really depends on the rarer bigrams, for which, in a short text, there are just 0 or 1, but the real expectation is somewhere in between.

H3 is generally ignored, but this is where languages can be most easily separated. Even different authors in the same language. But it will require multiple 100,000's of characters to get a good estimate.