When a user subscribes to Riverside using your affiliate link, Rewardful tracks that referral and adds 20% of their subscription to your pending commission total. When the commission total reaches a $100 threshold, Rewardful sends the money to your Paypal account. (Log in to Rewardful to change the Paypal account.)
The user you refer to Riverside must sign up within 60 days, before the affiliate link's tracking cookie expires.
As long as a user remains subscribed to Riverside, you can continue to earn a commission for the referral. There is no cap on total payouts.