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penchala rajupenchala raju 

What is system.Schedule()

NagaNaga (Salesforce Developers) 
Hi Raju,

Salesforce schedules the class for execution at the specified time. Actual execution may be delayed based on service availability.

You can only have 100 scheduled Apex jobs at one time. You can evaluate your current count by viewing the Scheduled Jobs page in Salesforce and creating a custom view with a type filter equal to “Scheduled Apex”. You can also programmatically query the CronTrigger and CronJobDetail objects to get the count of Apex scheduled jobs.

Use extreme care if you’re planning to schedule a class from a trigger. You must be able to guarantee that the trigger won’t add more scheduled classes than the limit. In particular, consider API bulk updates, import wizards, mass record changes through the user interface, and all cases where more than one record can be updated at a time.

Please see the below link for a sample requirement

http://salesforce.stackexchange.com/questions/37333/how-to-run-a-scheduled-job-every-15-minutes

Best Regards
Naga Kiran