Is it a modern concept to keep remembering them to 'learn' from them and honour the fallen?

Too my knowledge it was common in some parts of Europe, France and Germany, to build small memorials for local military dead; for example, from the Napoleonic Wars, and of course, Memorial Day in then United States after the American Civil War. As to modern memorialisation and remembrance-culture, that itself was very much a product of the aftermath of WWI.

How long will we continue to have remembrance ceremonies for it?

Since the instituting of Remembrance/Armistice Day on November 11th, in numerous countries that were involved in WWI, the date has come to encompass the dead from many conflicts since. As a Canadian, we remember those Canadians that fought in the Boer War, WWI, WWII, Korea, and in Peacemaking/Peacekeeping operations around the world.

In short, so long as armed conflicts continue to be fought, I'd imagine that those countries that recognize November 11th will continue to do so.