Michael Stone was convicted in 2001 and given three life sentences
The man who bludgeoned Lin and Megan Russell to death in Kent in 1996 must spend at least 25 years in prison before being considered for parole.
Michael Stone, 46, was given three life sentences in 2001 for the murders, and the attempted murder of Josie Russell.
The minimum jail term he must serve was set by a judge at the High Court.
Mr Justice Royce said he felt a "whole life order" was appropriate, but 25 years was the longest tariff he was "legally entitled" to impose.
He described the hammer killings in Chillenden, near Canterbury, 10 years ago as "truly appalling offences".
Dr Russell, 45, and her daughters Megan, six, and Josie, who was aged nine at the time, were attacked in a country lane on 10 July, 1996.
Josie suffered massive head injuries, but she survived and now lives in north Wales with her father Shaun.
Stone was originally convicted in 1998 of two murder charges and one of attempted murder.
Appeal judges quashed his convictions and a retrial was ordered, but he was found guilty in 2001 and had three life sentences re-imposed.
Stone has always protested his innocence, but a second appeal was rejected in 2005.
Stone murdered Lin and Megan Russell with a hammer
Before the murders, he had received support for mental health problems and drug addiction and spent time on probation.
A report released in September this year found failings in Stone's care and treatment, but said it was unable to say the deaths could have been avoided.
Mr Justice Royce set the minimum prison term on Friday.
He said he was "not legally entitled" to set it any higher than the 25 years recommended by the 2001 trial judge at Nottingham Crown Court and the Lord Chief Justice.
The judge said a period Stone spent in custody on remand would be deducted from the tariff.
"His case will have to be considered by the Parole Board and it is if, and only if, the Parole Board concludes that it is safe for him to be released that he will be," he concluded.