The longer the better - some people feel it right away, some don't. There is a reason Robb Wolf says to try it for 30 days. Most people will notice a difference by then if they are ever going to notice a difference. And 30 days is a long time, but most people can commit to it. If you tell people 3 months, well that's gonna put a lot of people off. And if you say 2 weeks, there are quite a few people that need a bit more time than that.

I agree with Dave S., though I'd like to add something. For all humans, and especially celiacs, gluten causes all kinds of intestinal damage. If you cut out wheat, barley and rye for a few days to a week then you stop creating new inflammation. The longer you go without gluten, the more of a chance your body has to heal. The haling, honestly, can take months.

When I first went gluten-free a few years ago (coupled with better exercise), I actually felt worse for the first week or so. I wasn't getting sick, but my body was beginning to flush out a lot of toxins. It wasn't until later, when I had healed some, that I felt the real benefits.

The numbers I've heard are that if your gut it sensitive to wheat that it take about 6 weeks to heal. That also why I'm against "cheat days" because aven a once-a-week exposure is enough to keep you permanently damaged.