In short, there is no expiry date on the apostille! Most apostilles are accepted for many years after it has been issued to your document. The apostille has an issue date, but it does not have an expiry date.
However, you may find that an organisation or government in another country ask that the apostille is only 3 or 6 months old.
This is common when the document in question is time sensitive. For example, documents confirming you are single, criminal record background checks and other documents that are factually correct at the time they are issued may only be accepted within certain time frames.