It depends on the fighter plane. Early model A6M2 Zeros had no defensive armor and enormous fuel tanks. They would explode with very little damage. By August of 1942, the A6M5 Zero pretty much replaced the A6M2. The A6M5 had a more powerful engine, limited amount of armor and self-sealing fuel tanks. They were slower and had only two thirds the range of the A6M2.
Early model Japanese Army fighters were very thin skinned and easy to destroy with light damage. They lacked any armor protection or self sealing tanks. Late model Japanese Army fighters, like the Ki-84, had armor plated seat backs and self sealing tanks, making them harder to shoot down with just light damage.
Early model Bf-109 and Fw 190 German fighters were fairly easy to damage, but by 1943 the Germans upgraded the armor protection of the Bf-109 G and Fw 190 A-5 to the point where they could pass through a combat box of B-17s and survive the damage inflicted by the machine guns shooting at them. They could do it two or three times, if they made a head-on attack, before the damage became serious enough to break off the action.
Before every mission, the Eighth Air-Force would brief the pilots and navigators on the target for that mission. At the end of these briefings, the group commander would tell them not to worry about the flak. This always elicited nervous laughter from the aircrews. Eighth Air Force bombers flew so high, at altitudes of 24,000 to 28,000 feet, it would be very difficult to see the incoming flak. The bombers did change direction very often to throw off the flak, but Germany usually had a lot of cloud cover and the flak was usually aimed by radar. After the bombers dropped their bombs they would change direction very often, but it was dumb luck to avoid being hit by flak and almost impossible to dodge in any premeditated way,