Be sure to follow a research plan so you don't constantly forget what you have done so far.
At some point your must FINISH researching and start writing something or advising your client.
Hopefully you always follow a research process and use a research checklist so you won't miss anything and can easily evaluate whether you have anything left to be done.
How to Tell When You are Finished
How to Ensure You Will Never Be Sure That You Haven't Missed Something Important