If copyright law is involved then you're looking at 70 years after the author's death or 95 years depending on which expires first for it to be public domain and only the original iteration is released into the public domain. That's for the USA and different countries have different laws and I'd imagine that would complicate things massively as an online library would have to be compliant with various laws of different countries

So take Mickey Mouse for example he's now in the public domain but only in his original form in the Steamboat Willie cartoon, anything that uses something more recent than that like his red shorts for example would be taken down.