Bharat, this help doc sums it up best:
https://docs.clay.com/en/articles/9654095-credit-rollover-after-cancelling-or-downgrading-subscription
Extra credits will remain until the end of the current billing cycle, at which point they will be capped at 2x of your plan. (E.g. because Free plan has max of 100, the rollover is capped at 200.)
You can see the billing cycle timing in Clay, under Settings > Plans & billing.