The bus route from Zagreb to Munich is among the most popular international routes from Croatia, trough out the year there are several daily departures between the cities. The bus travel time for the 550 km long route is between 8 and 9 hours.
In Zagreb to bus departure from the main bus station and in Munich the buses drive to the ZOB.
If you are planning a roundtrip you can same money if you buy a return ticket where the outbound and return trip is with the SAME company, you can see timetable from buses between Munich and Zagreb here