From my understanding canasa isn't for long term use, but people use it anyways to soothe any residual inflammation on the left side. The time period appears to be within 6 weeks, so I would expect to kick in around that time. When I used it, it was next to instantaneous.
From the site
pdf:
CANASA works at the site of inflammation.
In clinical studies, CANASA was shown to be
effective in relieving symptoms of UP, such as rectal
bleeding and stool frequency, at 3 and 6 weeks.
It is not known if CANASA is safe and effective
for use for longer than 6 weeks. It is not known if
CANASA is safe and effective in children.