For children who are drinking large quantities of milk, or eating primarily dairy or wheat-based foods, you may see subtle changes within a week, especially in younger children. For other kids, the change is not apparent until a few weeks later (usually after accidental ingestion, when there is a noticeable regression). However, many children become worse before they get better. This is probably due to a "withdrawal" reaction to the addictive, opiate properties of these foods. In our experience, it is a good sign, so stick it out!
Also, keep in mind that if a regression lasts more than three weeks, there may still be a source of gluten or dairy in the diet, which will prolong the child's discomfort, and your own. This is why it is so important to become educated about the diet, and how to do it properly.