Further to the point about multiple posterings of the same item; probably what happens is that your conscious brain stops seeing them anymore, yet it's still being absorbed. I believe several scientific studies have been done pointing to our unconscious absorption of messages embedded in advertising.
Yes, this is exactly how a lot of advertising works. Like, for example, the countless signs all over the world that simply say "Drink Coca-Cola." They are ubiquitous. You might see dozens of them in a single day. Similarly, the large billboard space that Sony has purchased along the Gardiner for decades; before being retrofitted with current billboard technology, it didn't even advertise a specific product. It just flashed "SONY, SONY, SONY, SONY . . ." over and over again in a huge 4x4 neon array.
Now you might wonder what the point of such advertising is. Who in the world hasn't heard of Coca-Cola? And do the people at Sony expect drivers to suddenly pull off the highway and buy a Sony product just because they see the company name flashing over and over again on the commute to work everyday? Of course not.
The purpose of this kind of advertising is to ensure that when you eventually do go into a restaurant or convenience store to purchase a soft drink, the very first choice that pops into your head is going to be Coca-Cola. When you go to buy a TV at the electronics store, you will immediately consider the one that has SONY written on it as a top choice.
This kind of advertising works because the products in question are very general in appeal. It is practically guaranteed that everybody who passes the Sony sign will be purchasing consumer electronics at some point. It is a virtual certainty that at least 90% of the people who see the Coca-Cola signs all day will purchase a soft drink sooner or later.
On the other hand, if you are the kind of person who never buys soft drinks, no amount of exposure to "Drink Coca-Cola" signs will ever motivate you to buy one. That is because, contrary to popular belief, advertisers have no power to create new behaviors. All they can hope to do is influence existing behaviors. If advertisers could create new behaviors, we would see ads like "BUY MORE CARS," "DRINK BEER," "SMOKE CIGARETTES." But advertisers don't want you to drink beer. They want you to drink Molson Canadian. They don't want you to smoke cigarettes. They want you to buy Player's Light.
Which brings us to the problem of so much postering. Like the previous Spanish lessons example, which I think was even more pervasive than the Coke signs, if that's possible. If it were likely that you, me, and the thousands of other people who see this message all day everyday were, at any time now, just about to go in for some Spanish lessons, then this postering campaign would be very effective. Whatever school puts these things up would certainly be my first, if not my only choice. The problem is that I have no real interest in learning Spanish, nor I feel safe in assuming, do the vast majority of people who see these posters. The advertisers seem to believe that they are able to create a behavior: the desire to "Think in Spanish." They do not have this ability. Many street posters share this fault.
So, the summary is that bombarding a general audience with the same simple message over and over again is an effective advertising strategy only if your goal is limited to influencing a behavior that you know your audience is going to engagen in anyway. If you think that you can use this method to create some new behavior, it isn't going to work.