Advertisements or promotional campaigns are planned for a certain period of time to make the maximum impact. Example is seasonal products whose advertisements go up as the buying season nears.
However, over time the advertisement loses its effectiveness because of a variety of reasons – most people have seen it, lost its relevance, is boring to watch after a few times and so on and so forth.
Half-life of an advertisement is used to indicate the period which it will take to decline to half its original efficiency. For example “two week half-life” means that it will take 2 weeks for an advertisement to reduce to half its efficiency level.
In the above picture, 2.5 weeks is the half-life of a particular advertisement.