My passport is expiring in Feb 2015. I went to BLS in May. I got it renewed in early June and updated the new passport info to CIC for my CEC application. The reason i did renew early because my WP is expiring in Sep 2014 and i wanted to apply for BOWP. I think if i would have applied BOWP without renewing my passport then my BOWP would also have the same expiry as my passport. So since i now applied BOWP using my new passport, i should get regular BOWP extension period.
So i think one can always go and renew their passport at BLS prior to one year of their passport expiry. Hope this helps!