Hi All .
How do I keep the retention of SSAS log for 1 month worth of log ?
You never mention which log or what version of SSAS so it's hard to say. Starting with SSAS 2012 SP2, the size and number of logs for msmdsrv.log is configured in the msmdsrv.ini file. The query log doesn't work the same way at all but the properties can be changed when connecting to SSAS with SSMS. You can find these in the following documentation:
Log operations in Analysis Services