I'm not certain, but I believe the CGS is only offered for 3 years terms (the 2014 CGS only granted 105K awards- see below). So, if you anticipate completing in Spring 2018 you may only receive fellowship funding until then, so a ~1.5 years.
http://www.sshrc-crsh.gc.ca/results-resultats/recipients-recipiendaires/2014/cgs_doctoral-doctorat_bes-eng.aspx