The truth is that it depends. Amazon runs a 2 question survey every day with their teams as they log in simply asking how the employee is and what their opinion of different issues are.
Some factors worth considering are performance and morale over time, significant change experienced in recent times, churn (turnover) rates and any potentially concerning metrics. These would suggest that bi-weekly, monthly or bi-monthly surveys should be considered.
We would advise against overly frequent large surveys as this tends to discourage responses. By the same token, annual surveys and feedback is just not enough. Employees should be heard on a regular basis. Short surveys can be conducted weekly, biweekly or monthly and larger engagement surveys every quarter would represent best practice.