Sorry it took me a little while to get back to you. I thought I'd read that it should be an hour, but what I read doesn't give a specific amount of time. (Do note that this guide, while I think it is excellent, was written by an engineer, not a dietician.)
An aside, which I find interesting: In other cultures, they eat breakfast, dinner, and supper, rather than the breakfast-lunch-dinner cycle that we're accustomed to in the US. (In other words, the big meal is in the middle of the day, not at the end.) There's also the saying, "Breakfast like a king, lunch like a prince, dinner like a pauper."
I know that darxus doesn't agree, but I think there's something to this (even though I often snack right before bed).