When you say Canna, do you mean PK? or just normal nutes? If just normal nutes then 8 days should be fine. The Stress Ripening is also a kind of flushing, the plant has no light, so it uses whatever food it has stored.

If the last time you fed PK was the 29th and you are cropping the 13th thats 14/15 days, which is just right for PK.