Just off the top of my head I think it's 75 years after the person dies, then it becomes public domain as long as nobody from the estate renews the copyright.
These days I'd double check just to be sure cause 75 years only takes you back to the 1930's. If it's actually public domain, I'm not sure about placing your own copyright... obviously you can't copyright the music, but maybe just the transcription?? Sorry, don't have a clue.

I'm just thinking about tunes like "Happy Birthday". It still has a copyright so that anytime anybody uses it they have to pay royalties. However I've never seen anybody copyright a transcription of the music except in the case of a book filled with musical scores, in which the entire book is copyrighted.