Runway's generative tools run using two types of credits — monthly plan credits and purchased credits. Monthly plan credits accompany a paid subscription, while purchased credits are obtained through one-off credit purchases starting at $10, where $0.01 = 1 credit.
Monthly plan credits, as noted on our pricing page, do not roll over. They reset within 24 hours of your billing day to your plan's full credit allowance or to 0 credits if you cancelled your plan.
All unused credits from your last cycle are forfeited on your billing day, so make sure to use them before this date. If you don't know when your billing day is, see here for instructions on how to determine this.
Purchased credits, meanwhile, are never removed from your account unless spent, and are not affected by your billing day or monthly plan credit reset.
Generations on your account will automatically deduct monthly plan credits before spending purchased credits. You cannot manually select which balance to spend from.
For more information on how credits function, please see How do credits work?