Previous roll outs have pegged the update at around 1.5GB, so definitely download the update over WiFi. The upgrade to Marshmallow doesn’t seem to have had such a large visual impact on the look of the Galaxy S5’s UI when compared with some of Samsung’s newer handsets. But the update does still include the main new Marshmallow features, such as Doze, granular app permissions, and Google Now on Tap, as well as the latest Android security patch.
The update has already been spotted landing on handsets in Sri Lanka. Given that the global model is available across so many regions, there’s no strict time frame for when the update will arrive for your particular handset. It could take days or more likely weeks for the update to get around the world, so don’t stay up looking out for that update notification or hammering the manual update check button.