Any duration dealing with physical actions is either doubled or halved, whichever is better for you. For example, if the typical person can hold their breath for thirty seconds, you can hold it for one minute. If the typical person can march for four hours without stopping, you can do so for eight hours. In terms of harmful effects, if a poison paralyzes its victims for one minute, you are paralyzed for thirty seconds. The minimum duration is always one round.