When should i change the transmission fluid in my 2004 chevy impala ss

IF anything - routine service only ~ drain fluid - replace filter and refill - ONLY! Do NOT have the transmission flushed!

As Pushrod said, especially in this case -- with so many miles, you can cause so many problems flushing fluid from a transmission. Even if you drain and fill the transmission, hopefully this has been done regularly in the past, as servicing the transmission after not being serviced for a significant period of time and mileage can result in debris freeing up with the transmission causing new problems. I only say that because of your 220,000 mile indication.