Its hard to tell how long it will take to see effects, if any are seen at all. Everyone's gut flora is different. Sometimes hi numbers of organisms are not necessarily better sometimes they are. VSL is better than culturelle b/c it contains a mix of probiotics instead of just L. Acidophilus GG. Probiotics are a great addition to UC patients but they are not a magic bullet and only have a natural way of reconfiguring intestinal flora. Sometimes numbers count and sometimes specific bacteria count, its largely based on the individual.
It also depends on many factors like: how much fiber and FOS containing foods you consume, your current pH in your gut, the amount of overgrowth, the type of overgrowth, overall diet, severity of condition, etc. How many organisms actually survive implantation only nature can divulge. Remember, we are taking something grown in a lab and throwing them in the deep end of the pool to see if they can swim, a tough adventure for any organism.