Wikipedia's Sobremesa article and La Tienda's own culture page both describe sobremesa as a living Spanish table tradition tied to agrarian life and the siesta, but neither names a year, century or era for when the custom of remaining at the table began.
What would resolve this A dated literary or historical reference (a period cookbook, a travel account, an academic food-history source) naming when Spanish sobremesa is first documented as a named custom.
OpenFood historySobremesa (Wikipedia)