>"Your session is about to end. You've been inactive for a while. For your security, we'll automatically sign you out in approximately 1 minute. You may choose "Stay signed in" to continue or sign out if you're done."
No stress put on the user with a timer (e.g. "You will be logged out in 3...2...1"). Instead, they simply state the facts in a calm way. "For your security" tells me why they're logging me out. Perfect voice for a bank. I want that level of simplicity in all my financial apps.