I assume you are asking about Job Logs that are generated by Project execution. Configuration for purging these logs is accessed under the Logs > Log Manager screen. Once on that screen, go to the Job Logs tab and set your Days to Keep Job Logs setting to the number of days that will work best for you.
Here is an excerpt from the help documentation available within the product
Days to Keep Job LogsBehind the scenes a process will run daily which uses these configuration settings to purge the logs.
The number of days to keep job logs. All job logs that are older than the specified value will be deleted automatically. A value of 0 (zero) will keep the job logs indefinitely. If a job created temporary workspace directories, they are also deleted at this point.
Thanks - Jon