Its more dependant on the handler and honestly the emotion level of the SAR.
The dogs sense it, and if you are involved in a very emotional SAR the dogs will literally not stop until you force them to. Most teams will go dawn to dusk with breaks during the day if conditions change, and to recharge the dogs so to speak. Let them out of work mode and let their mind relax a bit.
The teams may also be watching wind conditions. That is by far the biggest factor using airscent dogs. If the wind changes, it can cause a whole new set of smells to come into a search grid. Due to that, the best times to search are going to be during low wind opportunities.
Its a daunting task, especially when you really dont know any specific area to target.