SQL & PL/SQL

Hi,
I am searching for long to find a way to estimate the execution time for CTAS commands. I am a DBA. Our users run CTAS commands to load millions of rows. The commands fetch data from 4-5 very big tables each with millions of records and process them using where clause and group by clause and finally create the table. All these things are coded in the CTAS command. These CTAS sometime takes long time like 5 , 8 Hrs. Users frequently ask me to find how long it's going to take. I use both OEM and TOAD. But I couldn't find the time estimated from these tools. I feel that there must be some way, but I don't know the method.
Can any body please help me in this regard?

Thanks & Regards
Ananda Basak