I think the 4 second interval suggested by BMW is a bit silly. I switch the engine off at RR crossings or stoplights where the projected stop time is over 30 seconds. I follow the same criteria as EfficientDynamics:

1) Engine must be completely warmed up. We all know cold starts are bad.
2) The battery has a sufficient charge.

With the rest heat feature, shutting off the engine has few drawbacks in cold weather!