This depends on the the amount of your purchase and over how many instalments you choose to pay your plan.
Instalments are split into equal monthly payments and you will usually make your first payment to the merchant or retailer at the same time that you make your purchase. If your order is due to be shipped at a later date, your first payment will be when your merchant or retailer has shipped your goods.
As well as the first installment payment, you need to have at least the whole value of your purchase available as credit on your credit card as an authorization (hold) is kept on this for the duration of your instalment plan in order to guarantee future payments to the merchant or retailer.
The authorization amount reduces with each monthly payment you make until the balance is cleared and these authorizations are renewed periodically (around every 21 days or so). The previous authorization is removed as soon as the new one is obtained.