Undo Rollback time

this query will show you how much time remaining for your rollback session.

for 9i and up

SET lines 500 pages 500 col PCT FOR a5
SELECT state ,
undoblocksdone ,
undoblockstotal ,
cputime ,
undoblocksdone / undoblockstotal * 100 percent,
TO_CHAR(((((undoblockstotal - undoblocksdone) / (undoblocksdone/cputime) / 3600) / 24) + sysdate),'dd-mon-yyyy hh24:mi:ss') timetocomplete
FROM v$fast_start_transactions;
--just a note if your doing a parallel rollback the values does not give the correct values

for 10g and up

SELECT ADDR,
ROUND(((KTUXESIZ*
(SELECT value FROM v$parameter WHERE name = 'db_block_size'
)/1024/1024)),2) "Undo Size",
KTUXESTA "Status" ,
KTUXECFL
FROM sys.x$ktuxe
WHERE KTUXESTA='ACTIVE'