My experience was never any more than just a few days. They didn't stop eating until a bitch was in full bloom. They began eating again as soon as the bitches would no longer have been receptive. A slight weight loss would sometimes be noticeable but nothing dramatic.
IMO The critical time in the heat cycle doesn't last long enough for a healthy dog to harm themselves by not eating. Unless they were kept in a situation where there were always bitches in heat of course. Which would be a rather unlikely scenario.