gathering statistics completing in different duration.

Hi all,

Our DB size is 400GB and db version is 10.2.0.4.We have scheduled weekly schema stats  gathering by below query :

exec DBMS_STATS.GATHER_SCHEMA_STATS (ownname =><schemaname>, estimate_percent => '30', method_opt=>'FOR ALL COLUMNS SIZE 1', cascade => TRUE, degree=> 8);

but every time it is completing in diffferent duration like sometime it is completing in  3 hours or sometime in 4 hours.

kindly suggest if above query is correct or any reason for slowness.

advise is highly appreciated.

Regards,

Ashwini