14 days-correct answer: a. carla branch, personal trainer and marathoner, tells her clients it can take up to 14 days to adjust to temperature changes. when clients are preparing for an event that will take place in the heat of the day, branch coaches them to be active in the heat ahead of time: they have to try to get out in the middle of the day when its hot and exercise in order to acclimate to the conditions for the event. but remember, if you are just doing routine exercise, it is better to exercise outside when it is cooler, such as the early morning or evening.