Company culture and industry drive the decision regarding how often to conduct strategic planning meetings. Here are the three things to consider:
Rapidly changing industries: Some industries are moving so quickly that strategy should be reviewed more frequently. An example might be the technology industry.
New products/services: An organization introducing a new product or service might consider more regular strategic planning meetings. Another instance might be a company that is looking to grow via mergers and acquisitions.
Budget planning: Because strategic planning often raises questions about resource allocation, it's often combined with budget discussions.
There's no right or wrong when it comes to how often strategic planning meetings happen. The important thing is that they're productive.