I have read various answers to this question on the various detailing forums: 12 hours is the typical answer one finds. I wrote Collinite directly and asked them if there was an optimal waiting time before applying a second coat. I received this answer: a minimum of 30 minutes. Go figure.
BTW, the Collinite website recommends 60-85 degrees as the optimal temperature range for applying their waxes. "Using the product when the temperature is less than 60 degrees," it says, "could result in streaking or difficulty in removing the product from the finish." I had streaking problems last week when applying it at around 55 degrees. Maybe it was just me. Maybe I didn't let it set up long enough. I don't know. But I thought I'd pass the info on.