Scheme: change duration of NoteEvent

[Павел]

I want to write simple function that will be change duration of note. 
But I stuck on the first step: I can't "extract" (or "see") the duration 
property:

\version "2.20.0"
mynote = c'
\displayMusic \mynote
#(display-scheme-music
    (ly:music-property mynote 'duration))