Mitt Romney is not a declared candidate for president. The length after a declaration to the presidency is always roughly 18 months, but the portion of campaigns after declaration have been going on a few months longer recently as well. The earliest declared candidacy in recent memory is Hillary Clinton in the 2008 campaign. She declared in late January 2007, so if she had won the primary, she would have been declared for a total of around 21 months.
To answer this question, regarding continuous campaigns, I am going to use a college textbook on how to structure and run a political campaign. It is similar to Econ 101 or Public Policy 101 applied to the study of campaign strategy. The book is called Political Campaign Communication: Principles and Practice by Judith S. Trent and doesn't require any background to read. (I highly recommend :))
There are four stages of political campaigns:
3. Nominating Conventions
4. The General Election
A candidate for president may drop out at any of these stages and is running for president during the Surfacing stage, even though they have not "declared" they are running. Declaring is part of the Surfacing stage but it is not the only thing that occurs in the stage. He will restart the process of these stages after dropping out and go back to the Surfacing stage if she still has the will to run for the presidency. Surfacing is very confusing, so here is a more complete explanation: "the series of predictable and specifically timed transactions which serve consummatory and instrumental functions during the preprimary phase of the campaign." (It therefore does not include "grooming" candidates for offices)
These predictable Surfacing activities include: building political organization, fundraising, many kinds of speaking arrangements, awareness raising of the candidate, especially in a attempt to capture the attention of the media, conducting opinion polls to assess visibility and help devise candidate position on issues and platform, and creating campaign blueprints. In this stage, the candidate conducts polls and raises funds to attempt to find out if they have a possibility to win. In the news, you will often see someone like Mitt Romney say he is never considering running one day and the next say he might be running. This is the reason. The Surfacing stage is often used in small or local elections, but due to the costs and time of continuous campaigning that is limited to only higher office. In other words, the local mayor is not always campaigning, even though they use the same strategy.
This continuous strategy became the norm in the 1980s, so broadly speaking I think all candidates for president after about 1985 used a "continuous" campaign strategy, but as early as 1980. The historical reason was reforms to caucus rules of the Democratic and Republican Parties to increase more democratic participation, which started in 1976. In other words, instead of presidential candidates being picked by a handful of party insiders, candidates started being elected by the delegates to the conventions. Therefore, the candidates needed to be popular to more people to win the party primary. Also, these reforms weakened the power of the parties and left a power vacuum that was filled by pollsters, PAC's, and special interest groups. Special interest groups are made up of private citizens that are interested in an issue but do not support a party outright, so the party must offer support to them in order to secure their funding.
I think maybe Ron Paul? He started running for president in 1988. There are many examples of third party candidates having long histories of attempted presidential runs, but Ron Paul achieved national media attention and national ballot access more often. He also was a cross-over to the Republican Party primary. The question is not really answerable since we don't have access to individual politicians' records regarding their campaigns. They might not want us to know if they have been running for the White House for 15 years, but Mitt Romney is certainly not the first to have a long campaign. The official beginning of John McCain's (2008 Republic candidate) surfacing period can be placed at 1998 during his second reelection to the Senate when he declared a public interest in running for the presidency, but the actual beginning date of his Surfacing is unknown.
Reference: Judith S. Trent. Political Campaign Communication. Principles and Practice. 5th Edition.
"Ron Paul." Wikipedia.