Commissions last for one year (first year of the account).
For each successful payment we receive from customers you refer, we'll give you 20% commission, during the first 12 months.
Example:
Sam signs up as a paid customer on July 15, 2018, using your referral link.
You will earn 20% commission on all revenue we make from Sam for the period July 15, 2018 - July 14, 2019.