It very well could be parasite related. What you are taking is the standard basic cleanse. It probably wont cure you, but might help you figure out what is going on. It would be a good idea to watch your stool, and maybe even sift through it during the cleanse, otherwise it will continue to be an unknown.
You can expect to feel worse before you get better. Also you will probably have additional bowel issues, which is experienced by most people during a cleanse, so it is important to keep the bowels moving.
As far as the length of time required, this varies by person.