The duration for a flight from MUC Airport in Munich to MCT Airport in Muscat is about 6 hours and 30 minutes. However, you should keep in mind that there are some external influences that can cause a delay. In stormy weather, your flight from Munich to Oman accordingly take longer.
However, when it comes to the duration of the flight, keep in mind that you first have to get to the airport in Munich by S-Bahn. The same applies in Muscat, because from there you also have to get to the city from MCT airport.