[His declaration of a deadline surprises her, at first because she was expecting an answer now, but then due to how clear it is that this isn't an excuse... But a firm promise, decisive even while being a delay.
It reflects well on him. She knows from experience that some matters affecting many people cannot be decided at the very moment, and even if it happens to be true at that point in time, "I don't know" isn't an acceptable answer. Instead, "I will think"; allowing whatever necessary time to consider while still facing the problem responsibly and not running away.
Her face softens from the initial shock, giving him a warm smile-- proud and trusting, that he will find the answer he won't regret-- as she briefly nods]
...I understand. I will wait patiently, so please consider carefully what you truly wish to do.