In mid-November we saw a report that the LG G3 will be getting updated to Android 6.0 Marshmallow in December, specifically between the 16th and the 18th. It seems that the update has passed the testing phase as LG has already released the open source code for it.
The update itself is not out yet, but be on the lookout for reports from Poland, which is LG's usual market of choice for the first soak test. Note that this file is for the international LG D855 version of the G3, carrier and regional versions may get the update with a delay.
The LG G4 got its Marshmallow update a couple of weeks ago and it was a 1.55GB package.