This data from Lumen Research illustrates that there is a relationship between the number of seconds people spend looking at an ad and whether they can remember having seen the brand's advertising later on.
Seeing an ad for just 1" increases recall significantly and there is another big increase if the exposure lasts for 2". Recall increases further with longer exposures, but only a little. This means that if the advertising objective is simply to trigger brand awareness, exposures of just 1-2 seconds can be sufficient.
See 'The Challenge of Attention' for more details.