Hi,
Okay SHUTDOWN IMMEDIATE will take some time depending on the users connnected to the database.SHUTDOWN IMMEDIATE will take sometime to rollback the users transactions and if there are long running transactions it will take some time.Okay no probs it takes some time but while opening ur database ur database is in consistsent state and no recovery is reqd.
In case of any help please be free to ask me at [email protected]

Regards,
Rohit Nirkhe,Oracle DBA,OCP 8i
[email protected]