I assume that you are inquiring about professional fluoride applications. All the answers to your questions can be found in the American Dental Association's (ADA) Report titled, Professional topical fluoride: Evidence-based clinical recommendations. You can access this report and several other reports from the ADA at: http://ebd.ada.org/ClinicalRecommendations.aspx (scroll down to Topical Fluoride)
You will find the full report, an executive summary, and a very helpful chairside guide that can be copied for reference by all staff members in the office.
In summary the recommendations are based on age (<6 years, 6-18 years, or 18+ years) AND caries risk (low, moderate or high). A full explanation of the caries risk determination system is included in the report and is based on a list of individual risk factors as well as caries history i.e. the presence or absence of 1-2 incipient or cavitated lesions within the last three years. Highlights of the recommendations include the exclusive use of varnish for children under the age of 6 years, and a fluoride varnish or gel application every six months for all age groups at moderate caries risk, and a fluoride varnish or gel application every 3 or 6 months for all age groups at high risk with the exception of "only varnish" for <6 years. If you read the full report or executive summary and keep the chairside guide as a reference you will be fully updated on current clinical recommendations for professional fluoride applications.
Thank you for your question.