Check how older can we flashback To time.

select to_char(oldest_flashback_time,'dd-mon-yyyy hh24:mi:ss') "Oldest Flashback Time" from v$flashback_database_log;

Check how far back can we flashback To SCN.

select oldest_flashback_scn from v$flashback_database_log;

Check the flashback area usage.

SELECT * FROM V$FLASH_RECOVERY_AREA_USAGE;