People talk all the time about air changes per hour. Even a recent Nature.com article only discussed this metric. However, if you look at suppliers of heat recovery ventilation units, a different metric is liters/sec/person. About 10 liter/sec/person seems to be pretty good for commercial spaces. For home environments people can adapt to smells a little better, so the requirements can be lower.

Also: heat recovery percentages don't always include the energy required for forced air movement through the exchanger.