The preposition is optional there. Said that, it depends on the writer's style. 

[OALD][1] has an entry for it:

> *I've been waiting (for) 20 minutes.* 


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However, I did not have this idea unless saw that entry some months back. Still, I prefer using the preposition to satisfy myself! 

  [1]: http://www.oxfordlearnersdictionaries.com/definition/english/wait_1?q=wait