Buddy has been running off leash on the beach probably a couple hours after eating. Logic would say the risk of GDV (bloat), which is what I assume you're concerned about, would vary depending on what and how much they ate.
An article in the
August 2007 AKC Gazette looked at the role of exercise and food (among other things) in the risk of bloat. According to a 1998 study done by Lawrence Glickman, VMD, professor of environmental health at Purdue University School of Veterinary Medicine, "We found very consistently that dogs eating only dry food had higher risk of bloat. It didn't matter whether the owners added canned food or table scraps; as long as it was not dry-food-only, the risk went down." The study also says it was a common belief that drinking large volumes of water with meals and that exercise before and after meals were dangerous. Glickman found these to have no effect.
If I recall correctly, you feed raw which we know would digest more quickly than the dry. It would seem that in the wild a dog would naturally rest if it's stomach were full and run again when it was comfortable doing so. It would seem then that if you don't force Ruby to exercise, she should be fine to take out much sooner than the 5 hours your teacher is saying. Perhaps she is aware of some study we are not? If so, it would be nice to see it. Otherwise I wouldn't worry about it and follow your own instincts.