From Zagreb to Kotor in Montenegro, there is daily bus connection all year around, bus travel time is about 12-14 hours, depending on weather the bus follow the Croatian highway or travel trough Bosnia. Please note that the bus route might include a bus change at one of the stations, but do not worry driver will inform you if that is the case.
As the route is quite long, and it include several border crossing, minor delays may occur during summer season.