They could do it in a day; So why don't they? Israel hits the enemy with surgical accuracy. The only time civilians get hurt is when the terrorist conduct their evil deeds amongst the populist (which is quite common), or they deliberately hide amongst the civilians (which is another of their favorite tactics). They also love to launch attacks from Hospitals, Schools, and Mosques.
Fire breaks out in Devira Forest, north of Be'er Sheva; four firefighting crews on scene (Haaretz)
from the article: IDF: Gaza strike killed terrorist planning attack on Egypt border