How often should I distribute excess funds to other service bodies and WSO?

Your group should decide this, keeping in mind their prudent reserve requirement. Many groups have chosen to send donations quarterly. Often, donations are split, and OA suggests using a 50/40/10 formula, with 50% of contributions to intergroup/service board, 40% to WSO, and 10% to region. As with frequency of contributions, this is a matter for your group conscience.