the fire lasted for 20 days,(during that time)the whole forest was destroyed.

如果用which,那应该是during which,即the fire lasted for 20 days,during which the whole forest was destroyed.这句是非谓语从句，which 就整个指代了for 20 days。如用during which time，那么这个which即与time重复，或许你可以这么理解。

如果 用 which的话 那应该是 the fire lasted for 20days, in which the whole forest was destroyed.

大火持续烧了20天,(这20天里)整个森林被烧毁

这个不是定语从句啊……你看出来没？从结构结构上看，如果是定语从句的话that后面应该是没有那个time的阿……during that time就是在那段时间的意思啊……

我觉得是固定用法，没有见过用which的

背下来把，语言没这么多为什么，用多了就熟悉了。