Never. calibre is written in python, and Chromebooks only support Chrome webapps.
You can install calibre inside ChrUbuntu or Crouton, both of which enhance Chromebooks by giving you access to the underlying linux operating system (or dual-booting into another one) and allowing you to install "real" software.
Alternatively, instead of replacing the Windows computer with a Chromebook, keep the computer and replace Windows with Linux.
Linux isn't expensive at all -- it's free. Perfectly reasonable alternative to a Macbook as far as I am concerned.
