مُحْتَسِبًا means serving with sincere hope for Allah’s reward. Consistency in a good act is valuable, but allowing the desire for people’s praise to become the main motive weakens sincerity.
Example 1: A mu’adhdhin is punctual and careful in making the call.
Example 2: The service remains worthwhile even when no one praises it.
Example 3: The person avoids looking down on other callers or turning the service into a contest for fame.
سَبْعَ سِنِينَ is the particular duration stated in the text; it is not a personal test by which learners should pronounce judgment on someone’s sincerity or final outcome.