A while back I set this to 5 minutes, and tried it for a while, and decided I didn't like it, and set it back to never.
It turns out that many times I clock into a task in Emacs, but a lot of the work I am doing is not in emacs, so the idle timer was annoying me. Turned it off, if I look at it with customize-apropos org-clock-idle-time, it says it is set to never. I've grepped all my .el and .org files, and can find no references to that variable anywhere. However, it still fires off after 5 minutes. I come back to my emacs window to clock out of the current task and into another one, and I get prompted to decide what to do with my idle time. Any ideas where this setting is hiding? Thanks, Dave