To maintain Salesforce certification credentials, all certified professionals must successfully complete three online, release-specific exams within a 12-month period. Release exams are published in conjunction with major product releases (Winter, Spring, and Summer).
http://certification.salesforce.com/schedules

Three times a year (winter, spring, and summer), Salesforce releases new system feature updates.  In accordance with these updates, all certified professionals are required to maintain their certification by taking a maintenance exam every release cycle. 
 
Release cycles are about four months in length, and each release exam is available for two release cycles (about eight months). Refer to the schedule page on the Certification website for details regarding release exam completion deadlines: http://certification.salesforce.com/schedules. When the release exams are published you will receive an email with all the necessary details. 
 
If you do not pass the release exam by its deadline, all credentials maintained by that exam will expire, or in some cases become suspended. 

You will not be required to take the release exam for the same release cycle in which you first earned your credential. For example, if you were certified in Spring '14 your first release exam will be for Summer '14.

Earning a higher-level credential does not exempt you from having to take a release exam. You still need to take all release exams to maintain both your basic and higher-level credentials.

Release exams are 30-minute online and non-proctored exams that you can take at your convenience. Typically they are about 15 questions long, but can vary depending on the volume of new release changes. The exams are also open note, so you can refer to the online training and release notes during the exam. You are permitted three attempts on each release exam. 

There is an annual USD 100 maintenance fee for all Salesforce Certified individuals who hold the following credentials:Salesforce Certified Administrator
Salesforce Certified Force.com DeveloperIf you hold both credentials, the annual cost to maintain them is USD 200. The annual maintenance fee also includes the cost to maintain any higher-level credentials.
https://help.salesforce.com/HTViewSolution?id=000186547


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Thanks
Amit Chaudhary