Green’s Dictionary of Slang

24/24 adv.

[24 hours out of the daily 24]

(orig. US prison) all day, continually.

[US]Maledicta V:1+2 (Summer + Winter) 268: Other neutral terms are [...] 24/24 for all day, as in ‘Are you going to be in your crib tomorrow?’ Answer: ‘24/24.’.