Sirius – that’s a “how long is a piece of string” type question. There’s no correct answer and it depends on your server. Set the expire time to an hour, and if deleting expired files takes too long, change it to 30 minutes, then 10, etc.
Babak – don’t worry about it. It only exists if you already have wp-super-cache or another caching plugin installed.
Techseaport – it’s there alright. Not sure why you can’t see it.