Mine life is 21 years. This is because the NSW mining lease is max. 21 years.
The Nyngan resource is big and rich enough to extend the mine life and/or upscale production. The current scenario in the DFS only uses up 1/6 of the resource.
Max. mine life: 6 * 21 = 126 years Max. production: 38.5 * 6 = 231 tonnes Sc per year.
The combination of 115.5 tonnes Sc for 42 years would also be possible. Remember SCY also owns Honeybugle, which is similar in size and grade.