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A new pope could be elected as soon as the first ballot on Wednesday, or the process could continue for days. Since 1831, no conclave has lasted more than four days.

Up to four rounds of voting typically take place in a day. If no clear choice has emerged after three days, balloting is suspended for 24 hours to allow cardinal electors time to reflect. Another seven rounds of balloting then takes place, followed by another break, and so on.

If no pope is elected after 33 or 34 votes -- generally about 13 days -- then a new rule introduced by Pope Benedict XVI decrees the two leading candidates as determined by previous ballots will engage in a runoff vote. If the candidates are members of the conclave, they cannot vote in the runoff but are present for it. Whichever candidate receives the necessary two-thirds majority of the votes is the new pope.

Cardinal Jean-Paul Vesco, the archbishop of Algiers, told ABC News it would be "unexpected" if the conclave goes past Friday.

More than 30,000 people have gathered in Vatican Square for a glimpse of the smoke expected to emanate from the chimney of the Sistine Chapel following the official start of the papal conclave.

Black smoke would indicate that a pope has not yet been elected. White smoke signifies that one of the 133 cardinals has secured two-thirds of the votes to become the next pope.

It is unlikely that a new pope will be elected on the first day of voting.

The balloting process for the papal conclave may be shrouded in secrecy, but it is straightforward.

Each conclave member writes his choice on a paper ballot slip and folds it once in half. He then carries it aloft between two fingers as he walks to the altar.

The slip is then deposited into a special urn used only for the balloting process.

Conclave members are instructed to write their votes “as far as possible in handwriting that cannot be identified as his” to ensure anonymity.

Any conclave member who cannot make it in person to the Sistine Chapel due to illness or infirmity casts his ballot from this room in the Domus Marthae Sanctae. Those ballots are placed in a lockbox and carried to the Sistine Chapel.

Three scrutineers then count the votes by affirming what is written on each ballot and announcing it to the conclave, which allows the cardinals to record the votes themselves.

The first candidate to secure two-thirds of the votes is elected as the next pope.

Read more about the conclave here.

The prospect of an American becoming pope is "very unlikely," according to Dr. Miles Pattenden, a historian of the Catholic Church and a lecturer of history at Oxford University in England.

"There has traditionally been a lot of wariness about a pope from the Anglosphere," Pattenden said. "I shouldn't think that the current circumstances change that, especially now with the tensions between the Vatican and the Trump administration."

An American cardinal, Robert Prevost, has nevertheless started to emerge as a front-runner for the papacy, according to Father James Martin, a papal contributor for ABC.

-ABC News' Bill Hutchinson and Megan Forrester