各位老师，句子在下面：
Because of the long-haul flight, most visitors stay for longer (average 20 days) and want to see as much of the country as possible on what is often seen as a once-in-a-lifetime visit.
中文释义：
因为需要长途飞行，大部分游客会待得比较久(平均20多天)，会在这个“一辈子就来一次”的旅程中尽可能多地看看这个国家。
我有两个问题
1. "for longer"这个用法太奇怪了，for后面应该加名词的，我感觉for longer time是讲得通的，只写个for longer是啥鬼啊
2. "on what is often seen as a once-in-a-lifetime visit." 这块在句中的语法成分感觉分析不出来，on what是什么成分和用法啊。
Because of the long-haul flight, most visitors stay for longer (average 20 days) and want to see as much of the country as possible on what is often seen as a once-in-a-lifetime visit.
中文释义：
因为需要长途飞行，大部分游客会待得比较久(平均20多天)，会在这个“一辈子就来一次”的旅程中尽可能多地看看这个国家。
我有两个问题
1. "for longer"这个用法太奇怪了，for后面应该加名词的，我感觉for longer time是讲得通的，只写个for longer是啥鬼啊
2. "on what is often seen as a once-in-a-lifetime visit." 这块在句中的语法成分感觉分析不出来，on what是什么成分和用法啊。
