How long have "ads" been around in the modern form? By which I mean "visual content embedded in some medium that is viewed by significant numbers of people." If you include storefront signage in that then a long time, I suppose. But in terms of reaching anything like a mass audience I assume it essentially began with the advent of print publications. Maybe the time of mass visual advertising is just over? It reached its peak with network and cable television and radio, and now that those mediums are evaporating there is nothing to replace them that offers the same characteristics. Maybe the next model will be something entirely different.
The modern form of advertising however relies on a few specific developments:
1. The printing press. (1450)
2. Widespread literacy. (18th / 19th century)
3. Consumer products. (mid 19th century)
4. Consumerism, generally. (early 20th century)
5. Mass media. (early/mid 20th century)
Much of this converged in the 1920s as mass-market consumer culture really started to take hold, though it was interrupted by the Great Depression and WWII, re-emerging in the late 1940s and 1950s. The latter were the heyday of the large magazine publication (arguably the Web content of its day), with both nonfiction and fiction carried prominently. Both faded as TV replaced reading as a pastime.
Adam Curtis's 2002 BBC documentary, The Century of the Self details much more of the history of advertising and propaganda, particularly the role played by Sigmund Freud's nephew, Edward Bernays (who arguably made Freud as well).
