Last week prosecutors announced they would strike a deal with the lawyers of 20-year-old Melissa Reid, from Lenzie, East Dunbartonshire, to limit her sentence to less than seven years.
But Ms Reid, who was caught with Michaella McCollum, from Dungannon, Northern Ireland, has written in her prison diary that she expects a 15-year sentence, although it could still be as little as six years and eight months.
The women were stopped with 11kg of cocaine hidden in food packets in their luggage while trying to board a flight to Spain in August. Their families had believed they were still on the holiday island of Ibiza when they were arrested.
If their sentences are reduced, it is expected they will apply for permission to serve part of their sentence in a UK jail once they have paid the Peruvian state compensation for their crimes.
However, in her diary, Ms Reid said: "I have prepared myself for the worst, that being 15 years, because it's easier for me to cope with it that way, so if anything less is given to me it's a bonus."
It is believed no deal has yet been struck.
Last week chief prosecutor Gustavo Carvajal said: "The minimum sentence for drugs trafficking is eight years and the maximum 15.
"There is provision under Peruvian law for a reduced sentence but it is by no means a foregone conclusion."