The following view shows Data Guard standby lag via a histogram updated every second since the instance was last booted.
Note *: It appears this view is only available with Active Data Guard (DB open read only) – standard Data Guard in “MOUNTED” mode will not return rows.
select * from v$standby_event_histogram;
Note this guy / standby ain’t laggin’ much.
This view will also show you archivelog that has been applied.
SELECT sequence#, first_time, next_time, applied
ORDER BY sequence#;