That entirely depends on Kind of work you expect from the writer, for instance if you want him to do thorough research on your niche and niche is less popular or alternatively he has less experience in that particular niche, then he is bound to take more time in order to produce quality content. On the other hand, if he is experienced in particular niche he can produce content more quickly.
Like we have writer who some times produces 4000 words in a day when he is writing on his favorite niche and some times produces as less then as 2000 words only when he is writing on new topics. So choose accordingly.
Regards