It depends on what the agreement between the merchant and customer agreed upon. Always avoid payments. Set $ aside in the bank until you can buy it outright. Otherwise your throwing $ away.
Until the product is completeley paid for. It depends on how much that product cost when you are trying to determine how many months that you’ll be paying for it. For example: you have a product that is $100 and you have to pay $10 a month on it. If you pay $10 a month then it will take you ten months to pay the full $100. $10 monthly installments multiplied by 10 months = $100.