The following statement helps find how far back in time FLASHBACK can go.

select cast(
          from_tz(
                  cast( oldest_flashback_time as timestamp )
                 ,dbtimezone )
          at time zone 'US/Central' as date ) oldest_db_fb
     ,round((sysdate - oldest_flashback_time) * 24, 1) oldest_db_fb_hours
from v$flashback_database_log;