Como dizer "Jornada de trabalho 12 x 36" em inglês

I don't think this is a common enough labor regime in the US for it to have its own nickname or shorthand. You might explain it to someone like this: "12-by-36 means that I work 12 hours, then have 36 hours off, so I work the same 12 hour shift, just every other day."

Or more simply: "I work 12-hours-on, 36-off".

"X-on, Y-off" is taken in analogy to other common work schemes such as"14-on, 7-off", meaning 14 days on the job with 12 hours days, followed by 7 days off. There are other regimes like "2-on, 2-off", meaning two weeks working, two weeks off, and many other variations.

For other shift plan terms, this is a great wiki: https://en.wikipedia.org/wiki/Shift_plan

Other names for semi-common schedules in business settings:

"9-to-5" (nowadays "8-to-5") or "five-eights"- the standard 40 hour work week, with five days of eight working hours (usually not including lunch), and usually only called five-eights when discussing the other alternatives below.

"Nine-Eighty" or "9/80" - Work nine days per fortnight (Monday through Friday, then Monday through Thursday), 80 hours total, such that you still average 40 hours per week. This is my favorite one, as you get every other Friday off, and it's not as onerous as four-tens.

"Four-tens" or "Four 10s"- Work only four days, Monday-Thursday, for ten hours a day. Still 40 hours per week.