Run the same balance twice: minimum payments through an expiring promotional rate, a floating rate that ticks up and three skipped months, against an aggressive payoff. The gap between the two lines is the cost of drifting.
It depends far more on the payment than on the balance. Paying only the minimum on a high-rate card can take years longer than a payment modestly above it, and this calculator shows both timelines side by side.
The full penalty rate applies to whatever is still outstanding. Because nothing was charged during the promotion, people often reach the expiry with a larger balance than they expected.
More than the payments you skipped. Interest keeps accruing during a pause and is added to what you owe, so a three-month break can push the payoff date out by considerably more than three months.