You will reach a point in reading broadly when you

a) will feel that you have an understanding of the overall topic

and

b) you are prepared to commit tentatively to a position on the essay question.

This might happen very quickly and quite early in your reading, depending on the complexity of your essay topic; when it does you'll be able to go directly to more detailed or narrowly focussed texts with that point of view in mind. As you read more narrowly, you will need to make sure that your view is a valid one (and to fully understand and evaluate any other possible views) and you will need to gather enough information to successfully support the view you finally decide on.

When you are able to commit tentatively to a position, you should at least have identified the issues that you will need to read in more detail. For example, you may have read broadly for the question:

"How is the idea of womanliness constructed in the novel Emma"

and have decided that the following factors are implicated in any discussion or argument about this topic:

mannerisms
occupation
dress
behaviour
religion

This identification of issues to be researched will allow you to read more narrowly and in a way which will help you gather information around important themes. Don't read indiscriminately unless you have heaps of time!