The 15-year-old, from Bideford, Devon, was on a six-month "trip of a lifetime" in India with her family when she was killed.
Left in Goa in the care of her 25-year-old tour guide boyfriend Julio Lobo while the rest of her family went travelling, Scarlett's partially-clothed body was discovered on Anjuna beach in Goa in February 2008.
Mr Lobo was questioned by police but has not been charged in connection with Scarlett's death.
Initially Goan police said she had accidentally drowned, but following a sustained campaign by Scarlett's mother, Fiona MacKeown, a second post-mortem examination revealed she had been drugged, raped and killed. The cause of death was drowning.
India's Central Bureau of Investigation (CBI) took over the investigation and Samson D'Souza and Placido Carvalho were arrested the following month.
After being charged on five counts - culpable homicide, grievous sexual assault, destroying evidence, assault with criminal force with intent to outrage a woman's modesty and administering drugs with intent to harm - the pair were bailed.
The men, now aged 34 and 46, who have denied the charges, went on trial in Goa's Children's Court in March 2010, however less than a year later prosecutor SR Rivonkar resigned.
A new prosecutor was assigned and although some 30 witnesses - including Scarlett's mother - have now given evidence, many more are still to do so.
The prosecution is also waiting for permission to depose [examine under oath] via video conferencing, British man Michael Mannion, who claims to have 'witnessed' the attack on Scarlett.
Mr Mannion, who was living in Goa but has since returned to the UK, said he would not return to India because of threats made against him - because of what he saw.
"It's ridiculously frustrating, but no matter how long it takes, I'll keep fighting for justice because I have to," Ms MacKeown told BBC News.
"Going through the witness list, witness by witness, it's been estimated it could take another six years.
"In the meantime D'Souza and Carvalho can still carry on with their lives and that's not fair.
"I don't know if they think I'll go quietly away if it goes on and on, but I can tell you that's not going to happen."
In September, the latest hearing took place in the Children's Court to confirm Scarlett died from drowning.
"The trial has moved on - but it is moving at a snails pace," Ms MacKeown's lawyer Vikram Varma told BBC News.
"The perception with the Indian Criminal Justice system is that 'justice must be done', but the concept that it must be done in reasonable time is clearly an unacceptable burden to the state.
Mr Varma said all he and Ms MacKeown can do is "wait and watch".
The linearity of India's judicial process will ensure a verdict, the lawyer insists, although he fears the delay could be "many more years".
"Every prosecutor across the world knows that a delay in the trial is always an advantage to the accused," he said.
"Witnesses forget years later what they had seen or said - evidence becomes unreliable and doubtful and the benefit of doubt is always to the accused."
He said Ms MacKeown's determination to keep fighting for justice was admirable.
"She clearly has that extraordinary strength only a mother can have," Mr Varma added.
Ms MacKeown said: "It's hard all the time, but when it's really hard is when we all get together as a family and Scarlett's not there.
"This Christmas we're planning a massive family gathering - with a Â£5 secret Santa. It will be a nice, funny day - just the kind Scarlett would have loved."
Ms MacKeown, who said the decision to leave Scarlett in Goa is one she would "bitterly regret" for the rest of her life, is hoping to visit the Foreign Office with local MP Geoffrey Cox early in 2014, in the hope it can help to "keep the pressure on" the Indian judiciary.
Scarlett's mother said she also intended to return to Goa for the end of the trial - however long it takes.
She has always believed "a powerful drugs mafia" was behind her daughter's killing and the two defendants were "set up" as scapegoats.
"I hate the thought of them being jailed if they didn't kill her, but I have to believe the truth will come out," she said.
"I try not to let my expectations drive me mad, but I do have so much hope and faith that something good will eventually come from this," she added.
As Scarlett Keeling's family spend their sixth Christmas without her, their lawyer says the trial of two men accused of killing her is progressing "at a snail's pace".