Can I switch off disk while disk encryption is going on I am trying to encrypt a disk which is taking approximately 20-30 minutes per every 1% . 

Can I restart/switch off my Mac while the encryption is going on ? 
Would the Mac automagically resume encryption after switching it back on ?
If you're using the Lion's built-in encryption, the answer is yes. You can just work with the disk as usual, and the encryption happens in the background, even if you restart the Mac.
In Terminal.app, you can check the encryption status with

diskutil cs list