With flexible hours the average working hours per day are calculated. So if you work for 40 hours a month, a sick day gets you around 2 hours of sick leave.

edit: I think I got you now, you mean day labor, who get payed for how long they can work for changing employers?

I had to google for that, those are "Unständig Beschäftigte". Those are people who are employed for less then a week. They dont get sick leave. I couldnt tell you an example where it is allowed to hire someone like that. Even field workers/harvest helper get a normal contract with predefined working hours.