UNDO tablespace size

How to find out what does the size of the undo tbs need to be to satisfy both transaction and long running queries:

select
(select max(undoblks)/600 * max(maxquerylen) from v$undostat)
*
(select value from v$parameter where name='db_block_size')
from dual;

I got 14GB as a result :)
But so far is was ok to have undo tbs just 1GB of size

The maxquerylen is in seconds and undoblks is the number of blocks used within 10 minutes. So everything has to be taken to minutes and then multiplied.

To query the undo statistics use

select begin_time, end_time, undoblks, maxquerylen, ssolderrcnt, nospaceerrcnt from v$undostat

ssolderrcnt is the number of Snapshot too old errors and
nospaeerrcnt is the number of errors from transactions running out of undo space