According to the official website for the United States Postal Service, first-class international mail or airmail "delivery time varies by destination." However, USPS offers a Priority Mail International service that guarantees delivery between six and 10 business days.
First-class international mail can weigh up to 4 pounds but cannot exceed $400 in value. Delivery is available to over 180 countries. Priority international mail can weigh up to 70 pounds and is also deliverable to over 180 countries. For postcards and 1-ounce letters, a first-class global stamp can be used. If the Priority Mail International service is chosen, tracking information is provided.