3.69% of inflation occurs every year for Hex. That means 3.69% of the total supply is minted annually and adds to the total supply. These new coins are paid to stakers only (rather than miners in other cryptos). Because it's paid only to stakers, the interest percentage is a minimum of 3.69% ROI to stakers. It would likely be higher, as the percentage of total supply that is staked varies (reference chart below for total ROI for stakers).
The staker pool also receives 50% of all penalties paid as interest too.
The amount of interest your coins will receive depends on the amount of Shares you own in the Staker Pool. The total interest is distributed to everybody in the staker pool based on their percentage of shares.
You can see how interest your current stakes have accumulated under the “Staking” tab on go.hex.win