Just like any other (day) practice, it's too variable to say. A lot depends on the location and how busy it is, as well as whether it's an after-hours ER or whether it's a 24 hour facility. I've done relief work in several different emergency clinics -- usually there were 12 hour shifts for the primary DVM, but during busy spots they would overlap with a second (or third) doctor working a 6 or 8 hour shift. But other times (and clinics) there would be only 1 doctor on shift at a time, working 12 hour shifts.
I will say that you have to be prepared to work almost exclusively nights and holidays - certainly for the first while you're working there (and maybe always, if the clinics are only open "after hours").