''The famous artist will have been painting the mural for over six months by the time it is finished'' You already must have understood that "will have been ..." is the future perfect continuous tense. Now, to answer your question, it will be easier for you to understand if you first understand what the sentence is trying to say. The time talked about here is definitely the future. Let us take the time after six months. At that time, 1) The artist will have been painting for six months and, 2) The painting will have finished. So when you are talking about something finishing in future and are talking about the time when it will already have finished, you can use a sentence like this. Another way to depict some part of the same idea is, The painting will have finished by the time six months pass (Future perfect) For more information, refer to this webpage. This also has the example you have doubts about :) http://www.englishpage.com/verbpage/futureperfectcontinuous.html