see http://forum.avast.com/index.php?topic=57347.msg483820#msg483820

Does it delete the files created in it after reboot? I'm really excited about it..
It's deleted when the virtualized process terminates itself -- however, there's an exception for web browsers (see pic above); sandbox for web browsers can be deleted manually (in next avast version, you'll be able to schedule sandbox cleanup for web browsers).