Step 1: Time frame for Dr. Sadao's love.
Dr. Sadao did not have to wait long to fall in love with Hana. The story suggests that his love for her was immediate and natural. From the moment he met her, he felt a deep connection that evolved into love over time.
Step 2: Development of the relationship.
Though their relationship developed gradually, Dr. Sadao's feelings were not something that required years of waiting. His love for Hana seemed almost inevitable and intense from the very start, influenced by their shared experiences.
Step 3: The emotional bond.
The love between Dr. Sadao and Hana reflects a strong emotional and intellectual bond that grew over time, but it did not require waiting for an extended period.
Final Answer: \[ \boxed{\text{Dr. Sadao did not wait long to fall in love with Hana; his feelings were almost immediate, developing naturally over time.}} \]