How often to empty your menstrual cups depends on your cup size and your flow. When you first try a menstrual cup, you will realize pretty soon when to empty it.
The first day you wear it, check 3h later and that will give a good indication on how often you need empty it. The flow is usually different on each day of the cycle.
Personally, the first 2 days I put it in the morning, empty it around lunch time and then before bed. The remaining days, just morning and evenings are fine.
Everyone is different, get to know your body.
Knowing when to empty a menstrual cup is something that you will have to get a knack for as you get used to your body and the menstrual cup itself. Although you may not need to empty it for 6-12 hours, your flow, activity, the capacity of the cup, and your personal case impacts how often and when the right time is to empty the cup. You may want to pay attention to the clock, and check in on it every now and then to prevent accidents, and this will better help you gage the right time intervals to empty your menstrual cup.