The travel distance from Zagreb to Herceg Novi is 550 to 650 km depending on travel route; travel time with bus is about 12-14 hours depending on route. Buses are currently scheduled to operate whole year around.
As the bus has tree border crossing end to end, it can occasionally occur that there is long delay at the border which of course also will effect the arrival time of the bus in Herceg Novi.
You can see time table for bus from Zagreb to Herceg Novi here