根据剑桥语法书：We can put adverbs and adverb phrases at the front, in the middle or at the end of a clause.
Where there is more than one verb, mid position means after the first auxiliary verb or after a modal verb.
Adverbs of duration usually go in end position.
那道题里的 long 属于表时长的状语，放在第一个助动词之后 (has) 或情态动词后，或者句子末尾。