Coupon budgets are set and executed by number of redemptions. Amazon will take the coupon offline when the total number of redemptions reaches 80% of the budget you created.
Example: You create a $5 off coupon to offer the same discount on three different products (parent ASINs) and you set a budget of 2,000 units. For every single customer order of any of these ASINs where a coupon was used, your budget will be depleted by one unit. When the total number of redemptions reaches 80% of 2,000 (1,600), we will take your coupon offline. Keep in mind that for orders containing more than one coupon ASIN or more than one unit of the same coupon ASIN, the coupon discount will only apply to one eligible unit per order.