Calculate how much time/work is left to finish by RMAN

This command is specific to monitor RMAN activity but can be modified easily to monitor activity by other sessions

select SID, START_TIME,TOTALWORK, sofar, (sofar/totalwork) * 100 done,
sysdate + TIME_REMAINING/3600/24 end_at
from v$session_longops
where totalwork > sofar
AND opname NOT LIKE '%aggregate%'
AND opname like 'RMAN%'
/

　
or use  this

select OPERATION,MBYTES_PROCESSED from v$rman_status where status='RUNNING';