Initially the trial, which began in November 2012, was expected to end in early 2014. A spokeswoman said that this was no longer the case, and that the Munich high court hosting the trial will bring it to a close before the end of next year.
The judge decided that the case was going to take considerably longer than expected.
Zschäpe, 38, is on trial in connection with nine racially motivated murders and that of a German policewoman.
She is said to have been, along with Uwe Mundlos and Uwe Böhnhardt, part of the nucleus of the killer cell. The other two died in a murder-suicide in November 2011 after being surrounded by police in the wake of a botched bank robbery. It was only then that the cell was uncovered.
Four male co-defendants had helped the shooters by variously procuring weapons, giving them documents and renting vehicles, said the prosecutor. They have been identified only as Carsten S., Holger G. and Andre E. as well as Ralf Wohlleben.