My very active dad was on Irinotecan for 3 weeks. Despite being told to expect severe diarrhea, he was okay for the first two weeks and then experienced it during the third week. I guess it can occur early - within 24 hours of treatment and late - after 48 hours of treatment. However, they did equip us with this informational sheet that gave us a higher dose of Immodium to use than normal. This can help to control it. My dad ended up needing fluids in the hospital - but he had some other complications as well which may have contributed to things.
I hope Rick's experience is a good one with it. Just being aware of what to expect and knowing what to do when it does can help.