The estimated value of goods stolen in the Hatton Garden raid has risen to around £25 million, a court heard.
It is thought the prosecution is seeking up to £25 million from five of the men at the heart of the conspiracy.
Ringleaders John "Kenny" Collins, 76, of Bletsoe Walk, Islington, north London; Daniel Jones, 62, of Park Avenue, Enfield, north London; Terry Perkins, 68, of Heene Road, Enfield; and the group's oldest member, Brian Reader, 78, of Dartford Road, Dartford, Kent, pleaded guilty to conspiracy to commit burglary.
Collins, Jones and Perkins were each given a seven-year prison term for their involvement in the burglary over the 2015 Easter bank holiday weekend.
William Lincoln, 60, of Winkley Street, Bethnal Green, east London, was found guilty of the same offence and one count of conspiracy to conceal, convert or transfer criminal property.
Lincoln was also given a seven-year sentence.
Referring to the amount being sought in confiscation proceedings, Denis Barry, representing Jones at Woolwich Crown Court, said: "The Crown say £25 million between five."
The court heard that a full confiscation hearing is expected to last around six weeks, and a date was set for January 15 2018, with a deadline of April 30 that year.
Philip Evans QC, prosecuting, said: "This is an exceptional case which warrants an extension."
It had originally been thought that some £14 million worth of goods had been snatched during the raid at Hatton Garden Safe Deposit Ltd.
If members of the gang do not pay back what the judge rules they gained from the crime, they could face up to a maximum of 14 years of jail time being added to their sentences, without parole.
Tom Wainwright, for Reader, asked for a QC to be appointed to his client's case, stating they would have the required skill to deal with proceedings of this length and Reader's health.
He said: "The case is made more complex because Mr Reader is not in good health.
"He is likely to be in worse health come December or January."