Goliath's challenge

1 Samuel 17:14-16 - David was the youngest. The 3 oldest followed Saul, but David went back and forth from Saul to tend his father's sheep at Bethlehem.

For 40 days the Philistine came forward every morning and evening and took his stand.

I wonder why this went on and on for 40 days. Goliath came out twice a day (80 times in total) to challenge any Israelite to a duel. I wonder if he got tired of shouting at Israel. I wonder if his commander or his comrades had to coax him into doing it after the first week or two. Did either side expect something to change? Why did they just sit on opposite sides of the valley for over a month?

I wonder if David visited the battlefield more than once. If the book of 1 Samuel is in chronological order, then David was already one of Saul's armor-bearers. Perhaps he was not called into that service for this particular battle.

It's a little difficult to tell if this was his first visit. It says he went back and forth between home and Saul while his 3 oldest brothers stayed with the army. Yet when he decides to take on Goliath (sometime after the 40th day), it seems as though it was the first time he'd heard the challenge. Perhaps his trips to Saul weren't always to the battlefield. Maybe they were to Gibeah or some other base where Saul was tending to other business as his generals took care of the Philistines. Or maybe he had just never arrived when Goliath was taunting Israel.