Runway's generative tools run using two types of credits — monthly allowance credits and purchased credits. Monthly allowance credits are the credits that automatically come with a paid subscription as a perk, while purchased credits are obtained through intentional one-off credit purchase payments starting at $10, where $0.01 = 1 credit.
Monthly allowance credits, as noted on our pricing page, do not roll over. They are reset within 24 hours of your billing day to 0 credits if you cancelled your plan, or to your plan's full credit allowance value of either 625 (for the Standard Plan) or 2250 (for the Pro Plan) if you renewed. All unused credits from your last cycle are forfeited on your billing renewal day, so make sure to use them before this date. If you don't know when your billing day is, see here for instructions to determine which day of the month it falls on.
Purchased credits are never removed from your account unless spent, and are not affected by your billing day or monthly allowance reset.
When generating, Runway's credit system automatically spends any monthly allowance credits on your account before beginning to spend purchased credits. You cannot manually select which balance to spend from, but any available able-to-expire credits will always be spent before any non-expiring credits are.
For more information on how credits function, please see How do credits work?