Buying an article before seeing it

The sale is valid in such an instance. However, the buyer has a choice after seeing the article, if he wishes he can take it or he may return it. (Note: Buyer can also cancel sale prior to seeing article) (Kitaabul Buyoo).
There is no specific time period. This choice does not fall away until nullification is not found. The nullification will be anything which indicates towards or implies consent of buyer (Kitaabul Buyoo).