considerations to reset undo retention, your opinion

I have run a long PL/SQL DML statement in oracle 9i. It come across ORA-01555 snapshot too old error. Since I use Oracle automatic undo mode for undo tablespace. I plan to reset undo retention parameter to overcome this bottleneck. The original undo retention is 900 sec. I calculated optimal undo retention based on my undo size, db_block_size and undo_block_per_sec. End up, I got optimal undo retention number is 18571 sec. This is equal to 5 hours. If I set this new parameter for roll back segment in undo tablespace, what would be the impacts? Please tell me your ideas. Thanks.

You should change your code so that it runs faster, and does not process accross commits. Using pl/sql collections should help.