If you're not a young, active traveler, and your physical fitness is average, you're not too concerned about cost-effectiveness, and you don't need to see all the attractions, I recommend two days. Of course, arriving a day earlier and staying near the East Gate is ideal.
On the first day, it's best to arrive before 8 am to avoid tour groups. The mountain attractions include Tianzi Mountain, Laowuchang, Yangjiajie, and Yuanjiajie. I strongly suggest only visiting Laowuchang and Yuanjiajie. Tianzi Mountain has too many tour groups, resulting in a poor experience. Yangjiajie requires a hike to and from Wulong Mountain Village, and while the scenery is beautiful, it's nearly 5 kilometers and can be physically challenging. The reason for choosing Laowuchang is simple: fewer people and a superior experience. Yuanjiajie is the essence of the experience and a must-see. The first day's attractions cover the highlights of the mountain; the next day will cover the highlights of both the upper and lower sections.
On the second day, choosing the Ten-Mile Gallery and Golden Whip Stream is sufficient. For the Ten-Mile Gallery, it's best to hike in and then take the mini-train out. This avoids the crowds, as most tourists choose to take the train, resulting in long queues. The walking experience is far superior to the train ride. You could consider taking the train to save energy. There are many monkeys in Jinbian Stream, and the one-way trip is 5 kilometers, so there's no need to walk the whole way.