I think we first must know what they actually do in order to understand the masternode reward…
Masternodes allocate the task of validating the blockchain transactions and simply rewards nodes for their work. However the possibility of fraudulent action is a genuine risk and so masternodes require you to prove a level of commitment to the blockchain by locking a significant number of coins.
Each coin has its specifications and require a certain amount of coins what we call collateral. These coins are locked within a wallet, it can be the masternode itself or a cold wallet, and ensure that it is always in the interests of the holder of the node to not attempt to defraud the system. Masternodes are rewarded for existing and doing the work they exist for.
Let´s take an example from DASH. DASH need 1000 coins to be locked in order to create a masternode. You can see more details on masternodes.online.
I´m taking this example, but every single masternode is based on the same principle. Let´s say we setup a DASH masternode and start it. If you look on masternodes.online for the “AVG masternode reward frequency” you´ll see 8d 19h 4m 41s as the time of writing this post.
This avg timing is calculated based on the amount of DASH masternodes that are online, and we know that after starting a new masternode, it will take on average 3 times the avg masternode reward frequency before it gets rewarded by the first time. This fact is based on our knowledge and after setting up more than 3000 masternodes in one single year.
In this way we came to the conclusion that after each masternode Simple Pos Pool starts, you´ll have to wait 3 times the avg masternode reward frequency before you receive your first masternode reward, but don´t forget that we´re not dealing with an exact science. This is only a value that masternodes.online calculates based on the online masternodes and can vary.