Green’s Dictionary of Slang

boffo n.²

(US prison) a year (in jail).

1931		G. Milburn ‘Convicts’ Jargon’ in AS VI:6 437: boffo, n. A year. ‘He was handed ten boffos.’.
1950		Goldin et al. DAUL 31/1: Boffo. [...] 2. (P) One year. ‘Old John is in stir (prison) fifteen boffos and the Board hits him with three months for investigation. That’s a real lousing around.’.
1960, 1975		Wentworth & Flexner DAS.

In phrases

25 boffos (n.)

(US Und.) a 25-year sentence.

1992

Bentley & Corbett Prison Sl. 22: Quarter A 25-year prison sentence. (Archaic: twenty-five boffos).