SQL> desc v$undostat
 Name                             Null?    Type
 -------------------------------- -------- ----------------------
 BEGIN_TIME                                DATE
 END_TIME                                  DATE
 UNDOTSN                                   NUMBER
 UNDOBLKS                                  NUMBER
 TXNCOUNT                                  NUMBER
 MAXQUERYLEN                               NUMBER
 MAXQUERYID                                VARCHAR2(13)
 MAXCONCURRENCY                            NUMBER
 UNXPSTEALCNT                              NUMBER
 UNXPBLKRELCNT                             NUMBER
 UNXPBLKREUCNT                             NUMBER
 EXPSTEALCNT                               NUMBER
 EXPBLKRELCNT                              NUMBER
 EXPBLKREUCNT                              NUMBER
 SSOLDERRCNT                               NUMBER
 NOSPACEERRCNT                             NUMBER
 ACTIVEBLKS                                NUMBER
 UNEXPIREDBLKS                             NUMBER
 EXPIREDBLKS                               NUMBER
 TUNED_UNDORETENTION                       NUMBER

select u.undoblks*8192/1024/1024 undosize_mb from v$undostat u
where 
u.begin_time>to_date('27-01-2010 18:30:00','dd-mm-yyyy hh24:mi:ss') and
    u.end_time<to_date('27-01-2010 22:30:00','dd-mm-yyyy hh24:mi:ss');