It isn't so much a question of what focal length it becomes necessary as, as it is what situations it is helpful for. If you are running in to issues with motion of the camera body is causing issues, turn it on.
Quick handheld shots are possible at 300mm. (I've even done reasonably smooth video at 300mm handheld with IS.) Shooting from a steady position could be feasible at 600mm too I would think if you are really good and have your rig mounted to something like a shoulder mount. IS is a dual edged beast since it works against movement, but it also works against movement when on a tripod.
In general, I don't turn on IS unless there is a stabilization problem where it would be helpful. Normally with a tripod it isn't, but if there is a vibration it can deal with and has a mode that doesn't interfere with any motion I need to make, then it can be useful.