Terms of payment are negotiated by the buyer and seller depending on the countries, products, quantities, and their level of familiarity among other things. As a course of business, buyers may want to negotiate the longest time frame for payment possible. If you are dealing with a buyer that has not purchased from you before, you may try to negotiate pre-payment or partial payment up front. In this case, you or your bank may be asked to issue to the buyer an “Advance Payment Guarantee” to secure the deal.
Otherwise (or later on) Letters of Credit can be arranged with payment terms (including time frame, etc.) agreed upon by both parties.
Payment on “Open Account” basis is rarer (more common within North America) and not necessarily in the best interest of the exporter unless the business relationship is a strong and mature one.