AWR Reports

select snap_id, begin_interval_time,end_interval_time from dba_hist_snapshot;

@$ORACLE_HOME/rdbms/admin/awrrpt.sql

How to find the best Interval to Generate your AWR Reports?

To Single Instance this can be done using bellow query:

SET LINESIZE 200
SET PAGESIZE 200
UNDEF num_days
COL startup_time FOR a30
COL db_name FOR a10
COL snap_start FOR 9999999
COL snap_end FOR 9999999
COL start_interval FOR a25
COL end_interval FOR a25
COL range_interval FOR a40
COL qtd_snaps FOR 999

SELECT
    s.startup_time,
    di.instance_name,
    MIN(snap_id) snap_start,
    MAX(snap_id) snap_end,
    MIN(end_interval_time) start_interval,
    MAX(end_interval_time) end_interval,
    EXTRACT(DAY FROM(MAX(end_interval_time) ) - MIN(end_interval_time) )
    || ' Days(s) '
    || EXTRACT(HOUR FROM(MAX(end_interval_time) ) - MIN(end_interval_time) )
    || ' Hour(s) '
    || EXTRACT(MINUTE FROM(MAX(end_interval_time) ) - MIN(end_interval_time) )
    || ' Minute(s) ' range_interval,
    MAX(snap_id) - MIN(snap_id) qtd_snaps
FROM
    dba_hist_snapshot s,
    dba_hist_database_instance di
WHERE
    di.dbid = s.dbid
    AND   di.instance_number = s.instance_number
    AND   end_interval_time > DECODE(&&num_days,0,TO_DATE('31-JAN-9999','DD-MON-YYYY'),3.14,s.end_interval_time,TO_DATE(SYSDATE,'dd/mm/yyyy'
) - (&num_days - 1) )
GROUP BY
    s.startup_time,
    di.instance_name
ORDER BY
    startup_time ASC;