This article goes into some detail as to the flight crew duty hour limitations and rest period requirements. It is geared towards pilots but many of the same rules apply to cabin crew.
It provides the following table which shows the maximum number of hours that a duty can be, depending on its start time and the number of sectors involved:
Bear in mind this is the limit on the total duty period, which begins before the first flight pushes back (when the crew start their pre-flight briefing) and ends after the last flight comes to a stop (when they complete their post-flight paperwork). So you have to deduct around another 1.5 hours to find the maximum realistic duration between the departure of the first flight and the arrival of the last flight. For a typical 4-sector duty the practical maximum is thus 10.5 hours, and as little as 8.5 hours if the duty duty begins between 17:00-04:59 (hence it's unsurprising that the earliest flights don't tend to depart before 06:00 or so - it has a punitive effect on maximum duty length).
There are ways of extending this maximum duty period - such as having a third pilot - but that is only really worthwhile for long-haul flights and I'm not aware of any airline that does this for short haul flights.
The minimum rest period during a turnround is 12 hours, or the duration of the previous duty if that was more than 12 hours long.
So as
corporate-wage-slave has alluded to, if an aircraft is stationed somewhere overnight for less than 13.5-ish hours, it cannot be flown back by the same crew that took it there. That does lead to a degree of inefficiency in the flight duties, but evidently it's worth the additional custom that having a late departure or early arrival brings.