At this moment (2020), default earnings for 1 Epoch are 10.9 PRV. Most of the time you will see this amount added to your Node earnings after a period of earning. However, it is possible you earn a little less.
The short answer
Not every shard produces the same number of blocks, every Epoch.
The long answer
Let T be the total reward (PRV) and transaction fee (PRV, pETH, pBTC…) earned by all shards.
Let Ri be block_reward + transaction fee earned by shard i.
Let s be the number of shards.
The reward of a beacon = 2 * reward of a single shard
Let x be the reward percentage of Incognito DAO at the current year
The reward of the beacon/shard is split to each validator equally at the beginning of the next Epoch. Slashing will be applied in the future version. Epoch length is 350 blocks ~ 4 hours. The shard’s epoch is synced with the epoch of the beacon chain. Thus, a shard epoch may be shorter or longer than 350 blocks.