controlling APM access policy from iRule

Try this iRule instead, you must map ActivityTimeout as the Agent ID within the VPE. You also must attach the iRule to the virtual in the normal fashion.

when ACCESS_POLICY_AGENT_EVENT { 
    switch [ACCESS::policy agent_id] {
        "ActivityTimeout" {
            switch -glob [ACCESS::session data get "session.ad.last.attr.memberOf"] { 
                "*Standard_SSL_Users*" { 
                    ACCESS::session data set session.inactivity_timeout 150
                    ACCESS::session data set session.max_session_timeout 200
                    log -noname accesscontrol.local1.notice "01490000:7: [ACCESS::session data get "session.user.sessionid"]: [IP::client_addr]:[TCP::client_port]: --- Inactivity and Max timeout set ---"
                }
            }
        }
    }
}