There are, in essence, three parts to the impeachment process: the inquiry, the debate and vote on the articles of impeachment, and the impeachment trial. The inquiry could take multiple years; but that is unlikely. The other parts may take a month or so, each.
What happens if the legal process takes multiple years (like the Muller Report), and Trump leaves office? Will he be tried as a regular citizen?
Article I, Section 3, Clause 7
Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.
The primary goal of the impeachment process is removal from office. Should an official leave office at any time, prior to the conclusion of the impeachment process, there is no need to continue that process. It simply terminates.
However, that person may be still be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law regardless of how they leave. This process is separate from the impeachment process and any alleged crimes may be different than those alleged during the impeachment process.
Whether a person, such as Donald Trump, will be tried depends on the specifics of the alleged crimes and whether a grand jury and prosecutor are willing to bring any charges. Imagine the difficulty of meeting the requirements of Amendment VI: In all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial, by an impartial jury of the State and district wherein the crime shall have been committed, ...; particularly, when that person is a former president. [Emphasis added.]