The shortest possible answer is that it depends. It depends on the size of the application, technology used, experience and skillset of the pentester. You should also take into account, that sometimes also quality of the application also counts. On average one mid size application is tested in one week. We are following our own process of testing and assigning at least two skilled pentesters to perform testing. After testing presentation of results is mandatory and also verification that confirms, that all issues have been remediated properly.