NEW YORK (Reuters) - Five former employees of Bernard Madoff, convicted in March of helping the fund manager bilk investors of billions of dollars in his massive Ponzi scheme, will be sentenced this week, with prosecutors seeking prison terms of up to 20 years.

Like the six-month trial, one of the longest white-collar trials in recent memory, the sentencings will take time, stretching out over a week in four separate hearings.

The sentencings have been delayed for months, as defense lawyers fought the government's demand that the three men and two women should be ordered to pay billions of dollars in forfeiture they say they don't have.

First up on Monday is former back office director Daniel Bonaventure, followed in the coming days by portfolio manager Annette Bongiorno, computer programmers Jerome O’Hara and George Perez and portfolio manager Joann Crupi.

The five aides were convicted on all counts, including conspiracy and fraud charges, by a federal jury in March. Prosecutors said they knowingly propped up Madoff's fraud by creating fake documents and backdating trades.

The government has requested more than 20 years for Bonaventure and Bongiorno, more than 14 for Crupi and more than eight for O’Hara and Perez.

Defense lawyers have asked U.S. District Judge Laura Taylor Swain to show leniency. At trial, they argued that Madoff duped their clients into believing his investment advisory business was legitimate.

Attorneys for Bongiorno are seeking eight to 10 years, while lawyers for Bonaventure, Perez and O’Hara asked for home confinement or brief imprisonment. Cru pi's lawyers have also asked for a shorter sentence than the government requested.

Prosecutors also have demanded more than $150 billion in forfeiture. While the aides are unable to pay that amount, the size of the forfeiture could affect whether their relatives must relinquish certain assets stemming from their earnings at the firm.

Fifteen people have been convicted in connection with the fraud, estimated to have cost investors more than $17 billion in principal.

Madoff is serving a 150-year sentence after pleading guilty in 2009. His son Andrew Madoff died in September after a long battle with cancer. His other son, Mark, committed suicide in 2010 on the second anniversary of his father’s arrest.

Both brothers, who worked at the firm, always maintained they had no knowledge of the fraud.

(Reporting by Joseph Ax; Editing by Noeleen Walder and David Gregorio)