Typically they're pretty long and then after they fuse them to their hair they cut them to the length and style they need. Then after awhile they fall off naturally, or they can just cut their hair back to normal.
What I mean is how long will the extensions be left at for each movie. Are they just gonna cut off less and less?
Well in order to make them any longer they'd have to fuse extensions to the extensions.... or use fancy wigs.