I recently talked to a couple of inventors who were ready to file a patent application, and in both cases I thought it was too early for them to file. You have to be careful in holding off on a filing, particularly now that the U.S. has moved to a first-to-file system, but filing too soon can also be a problem. My answer to "when should I file?" is the following:
File as soon as you have developed a complete understanding of at least one version of the invention.
You want to get on file as soon as reasonably possible, both to protect against others who may be working on similar inventions, and to limit the window of prior art available to the patent office. That said, however, the patent rules require that the patent application be detailed enough to teach someone with the appropriate background in the field of the invention how to make and use the invention (the so-called enablement requirement). If you file too soon, and don't have enough information to meet that requirement, the filing date isn't going to do you much good.
Some people counsel waiting, even if you have enough detail to make a filing, so long as the invention is still being developed or modified. The reasoning is that the commercial version of the invention could look quite a bit different from the version you had in mind at the time of filing. I don't think that's a good reason to wait to file. While you're putting off filing, you are giving the patent office more prior art to work with and risking someone else filing a patent application for the same subject matter. If you have sufficient detail about the invention to file, then do it. If the invention evolves down a different path, you can always file Continuation or Continuation-in-Part applications to address the new versions.
The discussion above assumes there hasn't been any public disclosure of the invention. If there has been public disclosure, you have one year to file your patent application in the U.S. Most foreign countries don't give the one year grace period, so if you are considering foreign filings, you need to file your application before there is any public disclosure. Also, if you created your invention ten years ago and it has been sitting in your basement, then even though there has been no public disclosure you might be deemed to have abandoned the invention. Once it is complete, don't waste time getting your patent application on file.