he wanted to know how long the film had lasted when i reached the cinema

因为前面he wanted to know主句用的是过去式,
而后面从句he film had lasted when i reached the cinema又是一个现在完成时,
所以这种情况必须按照主从一致的规则,要把现在完成时变成过去完成时the film had lasted 可以改为 the film had started这里要注意last是一个持续动词，而start是一个瞬间动词 瞬间动词是不能用在一个时间段中的