In the Cardano block-chain, people can stake their ADA into stake pools and earn rewards. When will those people get the rewards?
Whether you receive any rewards at all depends on whether your stake pool generated any blocks during an epoch. An epoch is a set time-frame of 5 days on the Cardano block-chain. The likelihood of whether or not a stake pool is granted the opportunity to generate a block depends on how much stake the pool has, which is capped by the stake saturation point. Here are the details on a stake pool's expected number of blocks per epoch and how stake pool security impacts the generation of blocks and rewards. The flowchart below illustrates what happens between each epoch and when your rewards will be visible in your Cardano wallet.
Figure 1: General flowchart of the delegation cycle showing when to expect block rewards on the Cardano Network
At the earliest, your stake pool rewards shall be seen in your wallet in 15 days after you've staked, assuming the stake pool you delegated to generated blocks. It takes 3 full epochs to transition from Stake Pool Snapshot, to block generation, to rewards calculation, to finally delivering your rewards. You can track epoch transitions and their timing on any of the available wallets (Yoroi, Daedalus, etc.) and also some Cardano statistic webpages like pooltool.io.