Perhaps this most interesting question can be answered by looking at early German portraits, and asking the question: When did the custom of wearing ribbons instead of full medals became customary?
Going through my photo collection I have noticed that the earliest photos show only (high ranking) officers wearing Orders in their button holes. During and shortly after the Franco-German War of 1870-71 it looks like the first ribbons without medals are worn on the chest. Thus no medal ribbons in button holes, only full size decorations without the matching ribbons. Still, during the Franco-German War the EK2 is worn from the button hole (permanently on the kleinen Dienstanzug?). During the Great War I see the first ribbons without medals appear in button holes, also in combination ribbons, combining more ribbons into one.
Amongst the first ribbons to be permanently worn in the button hole is the Rettungsmedaille am Bande.
Some photos from my collection below.
Portrait of: August Wittenstein, Inf.Rgt. Nr.55 (dated Juli 1871)
Undated, photographer Vinmer, St.Quentin (Aisne) in France
Fritz von Bock und Polach (undated), photographer Ch.Boquet, Bar le Duc
Unidentified Lieutenant of the Reserve with ribbon bar
Beautifull incoloured photo, dated 19.1.1892 (???)