I would personally go with a much shorter "Are you still there? If you're not, we'll log you out to protect your account." The buttons speak for themselves and the more words you add to a message, the less of it people will read.

Hell, if one button says "stay signed in" and the other says "log out", I don't even know if I'd go with a message that long. Just "are you still there" may be enough, with a redirect to the "why did I get logged out" page if you let the timer lapse.

On the other hand, I want error messages to be as descriptive as possible, with a short summary for normal users perhaps. Operational messages may be short and sweet, but if I need to solve a problem, I've had it with the "oopsie whoopsie we made a mistakey wakey" messages apps produce these days.