Working hours — also called availability — control when clients can book a given instructor. They're set per instructor, not for the studio as a whole: a client can only book a time when the instructor who'd teach it is actually available.
Setting them
Go to Settings → Studio → Instructors, select an instructor, and open Availability. For each one you set:
which days of the week they work
a start and end time for each day
any breaks within the day
The default
Every new instructor starts on Monday to Saturday, 07:00 to 20:00. That's a placeholder, not a recommendation — set it to what's actually true before you share your booking page, or clients will be offered times when nobody is there.
How this fits together
Availability is one of several things that decide whether a client can book a given slot — the others are your studio-wide booking rules (lead time, booking horizon, cancellation window), buffers on individual appointments, and any blocked days. See Setting when clients can book for the full picture, including a step-by-step checklist for "why can't a client book this slot."