Credits don't roll over month to month, but they do have a fixed validity period that's longer than a single billing cycle. As long as you use them within that window, they remain available, including after a subscription ends.
Subscription credits

• Project Dream Lite (750 credits): valid for 60 days after purchase

• Project Dream Medium (2,500 credits): valid for 120 days after purchase

• Project Dream Pro (750 credits): valid for 30 days after purchase

Credit packages

• 2,000 credits: valid for 60 days after purchase

• 5,000 credits: valid for 90 days after purchase

• 10,000 credits: valid for 120 days after purchase

You don't need an active subscription to spend remaining credits, so unused balances can be used up after a subscription ends, as long as it's within the validity period.