The substance of the articles of impeachment is being developed everyday by the behavior and statements of President Trump, the administration and the reported testimony of witnesses before the Intelligence Committee. It is useful to recall the spring and summer of 1974 when the Nixon articles of impeachment were approved by 70 percent of the members of the Radino Committee (Judiciary) on July 27. The investigation of the previous months and the daily news stories documented the specifics for the articles. (I was at Georgetown Law School at the time.)
Nixon Articles of Impeachment, July 1974
Obstruction of justice (cover-up, smoking gun)
Abuse of power (use of IRS, FBI, others)
Contempt of Congress (ignoring subpoenas)
Trump Articles of Impeachment, October 2019
As of today, those same three articles are being discussed with the current factual base, starting with the Mueller Report, and now driven by the Ukraine whistleblower disclosure. Democrats will likely add a violation of the emoluments clause.
There is a lot of material. Democrats must decide how far a field do they want to go and how quickly do they want to move. A public process with the Judiciary Committee is next after the investigation, so before the holiday seems unlikely, but before February 3, the Iowa Caucus, might be their preference. Timing and subjects, many decisions, interesting moment.
The Nixon resignation timeline ran for two years, starting with the break-in in June 1972 and ending in August 1974.