From the docs, here are the limits for the debug logs:
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The following are the limits for debug logs: Once a user is added, that user can record up to 20 debug logs. After a user reaches this limit, debug logs stop being recorded for that user. Click Reset on the Monitoring Debug logs page to reset the number of logs for that user back to 20. Any existing logs are not overwritten.
Each debug log can only be 2 MB. Debug logs that are larger than 2 MB are reduced in size by removing older log lines, such as log lines for earlier System.debug statements. The log lines can be removed from any location, not just the start of the debug log.
Each organization can retain up to 50 MB of debug logs. Once your organization has reached 50 MB of debug logs, the oldest debug logs start being overwritten.
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So they aren't stored for a number of days, instead there is a finite amount of space to store them. Once they are removed they are gone for good I'm afraid - if you need to view them later you have to download them.
--- snip ---
The following are the limits for debug logs:
Once a user is added, that user can record up to 20 debug logs. After a user reaches this limit, debug logs stop being recorded for that user. Click Reset on the Monitoring Debug logs page to reset the number of logs for that user back to 20. Any existing logs are not overwritten.
Each debug log can only be 2 MB. Debug logs that are larger than 2 MB are reduced in size by removing older log lines, such as log lines for earlier System.debug statements. The log lines can be removed from any location, not just the start of the debug log.
Each organization can retain up to 50 MB of debug logs. Once your organization has reached 50 MB of debug logs, the oldest debug logs start being overwritten.
--- snip ---
So they aren't stored for a number of days, instead there is a finite amount of space to store them. Once they are removed they are gone for good I'm afraid - if you need to view them later you have to download them.
Please go through this URL:
https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_debugging_debug_log.htm
Thanks,
kathir