Answer: A strong desire is the desire when a person does not need a lot of time to return to the spiritual thoughts. See Baal HaSulam, Shamati No. 19, “What Is the Creator Hates the Bodies, in the Work.”
It is essential to try to acquire a strong desire to overcome one’s egoism and reach the property of bestowal. The strength of the desire is measured by the amount of delays and stops, meaning the amount of time that passes between having acute desires or, in other words, the length of a descent, falling out of spiritual efforts: a split second, an hour, a day, or a month.
Then one again resumes his work on overcoming his egoism and tries to reach bestowal. A strong desire is the desire when the interruption of efforts does not take a lot of time for a person, and he immediately awakens for the work. In this case, his efforts accumulate, as if into a single blow.