Lexercise therapy is designed to be a three-way partnership among the child, the parent(s) and the clinician. With this level of parent involvement, progress is typically much, much faster than in traditional school or clinic models that don't involve the parent in therapy sessions. Generally speaking, it takes at least three months of this kind of "family-centered," intensive therapy before re-testing shows a significant gain. That said, nearly all parents say they notice very clear changes after 4 weeks.
Naturally, part of the formula will depend on the severity of the child's processing problems and the age of the child. For a young child (e.g., under 3rd grade) it is rare for the process to take over 6 months.
Here is another way to think about how long therapy will take:
There are 32 word analysis levels that need to be covered before the child is in ~8th grade. But the most basic structures (that would permit the child to read almost any phonetically regular English word) are covered in Levels 1-21, and this is a good interim goal for a child under 3rd grade. On average, we can cover about a level every week. At that rate, assuming the child has to begin at Level 1, it would take about 21 weeks to cover the first 21 levels.
After the first 3 month period it is usually very clear to parents if the child needs to continue for another 12 weeks or if it makes sense to at least take a break from intensive therapy. The comments here, for example, were posted after these families had finished ~12 weeks of therapy.
The goal of therapy to to enable the parent and child to become independent in using a structured language (Orton-Gillingham) approach for teaching and learning new words and text levels.