Ganache can be kept in the fridge for about a week. You can reheat it in the microwave, but only heat at about 20-30 second intervals and at 50% power. If it heats unevenly, the ganache could separate. Stir well after each round in the microwave.
This recipe makes a lot of ganache, but you'll have a bit extra to pour over the cake balls plus you can use any leftovers to pour over ice cream!