A fixed amount (30%) of the block reward (15 XZC with current block reward) is paid to Znodes. Znodes are put in a queue and take turns to receive this block reward in return for hosting a Znode that hosts a complete copy of the blockchain and helps store and propagate blocks to the network.
Once they are in the top 10% of the queue, there is a degree of randomness in being selected as it uses the block hash entropy to pick the winner deterministically.
The frequency of the block payout depend on how many active Znodes there are. The more Znodes there are, the longer it takes to receive the Znode block reward.
To get a more in-depth understanding of the process, please read our article Understanding Znode Payments.
There are third party tools that estimate the frequency of Znode block rewards payouts and other Znode statistics: