Research can be almost endless. There are some subjects which could take a lifetime to research, and you still wouldn't be finished. So, how can you tell when to stop?
First of all, if the amount of material seems overwhelming, you have probably chosen far too broad a subject. Your topic should be narrow enough that you can locate and read most of the material written on the subject before it is time (remember your timeline?) to start writing the first draft.
Your research should include at least the following general areas:
These parameters will probably be different if you are doing historical research. You may have to go back 100 years in the indexes.
In addition, you should probably interview some people about the topic and visit some relevant locations. You will then have quotes and personal observations to add to your research.