During the long war between the house of Saul and the house of David, what happened to David?

Answer: He grew stronger and stronger

The war was protracted, but "David grew stronger and stronger, while the house of Saul grew weaker and weaker" (2 Samuel 3:1).

“Now the war between the house of Saul and the house of David was protracted. And David grew stronger and stronger, while the house of Saul grew weaker and weaker.”— 2 Samuel 3:1 (BSB)