Detailed Instructions: In this task, you're given a passage, further information available on a particular linked term from the statement, and an answer term. Your job is to generate a question that can use the information provided to obtain the given answer. You should use the information on both passage and link information to create the question. Note that the answer to the question should be exactly the given answer, and if the answer is none, the answer to the question shouldn't be obtainable from the passage or linked information.
Q: Passage: In January 1996 former National Geographic photographer Charles O'Rear was on his way from his home in St. Helena, California, in the Napa Valley north of San Francisco, to visit his girlfriend, Daphne Irwin (whom he later married), in the city, as he did every Friday afternoon. He was working with Irwin on a book about the wine country. He was particularly alert for a photo opportunity that day, since a storm had just passed over and other recent winter rains had left the area especially green. Driving along the Sonoma Highway (California State Route 12 and 121) he saw the hill, free of the vineyards that normally covered the area; they had been pulled out a few years earlier following a phylloxera infestation. "There it was! My God, the grass is perfect! It's green! The sun is out; there's some clouds," he remembered thinking. He stopped somewhere near the NapaSonoma county line and pulled off the road to set his Mamiya RZ67 medium-format camera on a tripod, choosing Fujifilm's Velvia, a film often used among nature photographers and known to saturate some colors. O'Rear credits that combination of camera and film for the success of the image. "It made the difference and, I think, helped the 'Bliss' photograph stand out even more," he said. "I think that if I had shot it with 35 mm, it would not have nearly the same effect." While he was setting up his camera, he said it was possible that the clouds in the picture came in. "Everything was changing so quickly at that time." He took four shots and got back into his truck. According to O'Rear, the image was not digitally enhanced or manipulated in any way.
Link Information: In 1971, National Geographic magazine hired O'Rear to document the lives of Russian villagers Answer: 24
A:
How long did the photographer who lived in St. Helena California work for National Geographic?