Two kinds of Credits, two rules:

Each subscription Credit grant expires 30 days after its period starts, or at the end of that paid period if it ends sooner. Renewing or upgrading early does not extend older Credits: the new grant has its own deadline. Returned Credits keep their original expiry, including when you kill an older preview. A coupon that grants a plan follows the same rule.

Credits you buy as a top-up, or receive as a straight credit grant, live in a protected bucket that never expires.

The Free plan has no monthly grant. Welcome Credits land once, are never topped up again, and do not expire while you stay on Free. If you move to a paid plan, they follow the subscription rule above from then on.

Your plan's Credits are always spent first, so the bought ones are the last to go. You never lose a purchased Credit to a monthly reset.

A yearly plan is paid once a year but delivers its Credits every month, on the same date as you bought it. Each month's Credits follow the subscription rule above: they are good until the next month's arrive (30 days at most) and do not roll over. A yearly plan never lands a whole year of Credits at once.