How much time and effort may have been put in?

[oshfdk]

Perhaps the most relevant example is the Voynich community itself. Many people set out to research and solve this manuscript, spending hours, days, weeks, months, sometimes years, and sometimes decades putting in 1000s of hours of effort in. Is it so unbelievable that someone may spend an entire year's worth of their free time writing a book meant to be unreadable, just for the sake of it? We tend to assume that if there is meaning in the text then there must have been a use case, but what if the author simply enjoyed writing weird/fancy books?

I see no real hard limit. If we use as the benchmark some modern hobbyists, then even spending 30 years on something like this is not off the table.

As for the deciphering/decoding effort, it took some 500 years to decode book III of Trithemius Steganographia. So no hard limit either on how hard the book should be for the potential decoder. And, as far as I understand, Steganographia had no purpose other than to showcase the cipher.