How often should appraisals take place?

1. Quarterly review.

2. Half yearly review.

2. Annual appraisal.

Well Appraisal offers a valuable opportunity to focus on work activities and goals, to identify and correct existing problems, and to encourage better future performance

So By calling something an appraisal is likely to give it certain "mystical powers" like pay rise, disciplinary, form filling bureaucracy etc depending on the nature of the organization.

I would still suggest that Informal and formal should take place every month.

annual/6 monthly interim is plenty SO LONG AS channels of communication remain open throughout the year.

Twice in a year - Mid year and Annually - Good practice in the industry

Annual or Biannually... The framework and frequency of reviews both are of equal importance. Most companies tend to hold appraisals once a year. Appraisals can be segregated into several types such as:

Reward Appraisals

Performance Appraisals

Development Appraisals

Performance and Development appraisals should go on constantly.

Agreed with Mr. Saiful Islam Hiron.

Final appraisal should be on an annual basis. Maximum frequency can be twice in a year i.e. after06 months. Performance review can conducted on quarterly or even monthly basis to give continuous feedback to employee. Conducting performance more frequently can help to identify improvement areas and take measures to improve performance when required rather than being too late.

Appraisal should to be annually

But actually it should be continued during employee service like daily with every action

also we recommend to be monthly and make award monthly for best employee according to that beside annual appraisal

there are many ways : 

Daily 

Weekly 

Monthly 

Yearly . 

Keeping records and setting goals for employee and then you measure it . 

Every Year

every6 months

Annual Appraisals are important

From the Employees Point : It should be twice an year as it acts as the motivating factor both monetary and Personal Development.

From Employers Point : It should be twice an year as the Employer would be saving a lot of money as he is paying the lesser money in comparison to the annual appraisal.

Actual : The HR Person will have extra work to do if it is twice an year/ Quarterly.