when should I stop searching the literature?
The facile answer is you don't stop searching the literature. Even as the review evolves, you should be including new references if they are noteworthy in addressing questions within the field. This process should continue as long as you are working in the field of the problem.

Of course, from a practical standpoint, you do need to select a cutoff. There should be a reasonable point in time in which you've set the outline of the review, and decided on the main topics and questions to be discussed. At that point, it would be fair to set aside adding more references, and stick to what's been published. However, you should continue to monitor the field, and if further revisions or updates of the text are necessary, then you should include the papers published in the interim as part of your updates.