When I was in grad school, Percival Everett visited for a Q & A session. This happened years ago, and I've since forgotten nearly everything from that session, but I do remember asking when he knew he was done researching for a story. His answer was, "You just know."
It was a little underwhelming at the time. Recently, however, I've realized you really do simply sense when enough is enough. I've had an idea for a novel in my head for the last year, and have been researching in earnest for the last few months. I've done research projects in the past that produced great volumes of material with little actual progression. We know that kind of research, the one where you think you're being productive but are actually just procrastinating.
I knew I didn't want to get into another quagmire, so I'd decided that research would be finished no matter what by New Year's Eve. It was documentary screening too, each program usually lasting about an hour, so even though the list was long, I knew I had X number of hours that could be scheduled over Y number of days. This could be done.
A couple of weeks ago, I knew I had to stop. There are two things that'll tell you when you've reached the finish line.
First, are you bored?
I researched wars, natural disasters, you name it. I even had a documentary series on my to-do list featuring interviews with the last World War I veterans. I'm sure the material I had yet to screen was fascinating, but I was simply worn out. If I wasn't doing my day job, I was at home screening. My weekends were swallowed up doing this. I was sacrificing my personal life, and professionally, I wasn't coming up with any new stories.
Second, do you have enough?
Each program I screened yielded two to five pages of notes. Longer and more informative programs could be three times as productive. Each piece alone doesn't seem like much, but then I looked at The Document and history was the largest section; somewhere in the neighborhood of 160 pages with about 120 interviews directly quoted. If each page of notes translates to two pages of story, that's a novel right there purely on regurgitation.
I'm sure there will be follow-up research, ideas will pop up in my head and I'll have to go back and look at some topics a little more thoroughly. But I at least sleep easy at night knowing most of the heavily lifting is done...and because I'm exhausted.