Your referral link is permanently valid, but the tracking relies on a 30-day cookie window. A user must sign up within 30 days of clicking your link for the referral to be attributed to you.
As long as a user signs up through your unique link within this 30-day window, their first 6 payments will be credited to you. There is no time limit on when those payments occur. You can share your link once, and you will continue earning whenever the user makes a payment, even if it happens weeks or months later.