入境前多久签证？有规定吗？

一年多次入境的签证，比如B2，应该在入境前多长时间去签证？据我所知，以前一次入境的签证一般只有三个月的有效期，也就是说拿到签证后三个月之内必须入境，否则签证失效。这个也适用于多次入境的签证吗？我给父母预约的是5月24号，本来打算9月份来，不知道可以吗？我在美国大使馆的网页上只找到这个：
How long before my planned travel date should I apply for a visa?

Because of significant demand for U.S. visas, there is usually a wait for visa interview appointments. If you need a visa interview, you may have to wait a few weeks for an appointment. Additionally, post-interview processing may take another month or two before the visa can be issued. Therefore, we encourage all visa applicants to call the China-wide Visa Information Call Center at 4008-872-333 well in advance of their anticipated travel plans. If you are a frequent traveler or someone who may be required to travel on short notice, we encourage you to keep your visa valid and apply to renew your current visa before it expires. Please plan ahead and apply early! Thank you for your cooperation.