Actually, that's not really 15 hours of programming a day.
This period is called "piscine" (swimming pool). That's the same "marathon" as the one you face when you're an epitech student.
During your first month at school, you'll learn, from scratch, how to use a UNIX shell and C programming by coding various things, such as some str*() functions and implement various functions for linked lists manipulation. If you've never programmed before, you may spend ~10 hours on this. Else it will just take a few hours. 15 hours is because some exercises can change at the last minute, just before the deadline which is around midnight every day (for example, at the last minute they may ask you to return 42 instead of 1 in some function, and the automated checking script will check whether you return the right value).