It's a decades old concept in other marketing/media channels, but for some reason some people have difficulty relating it to the Web. If exposure to advertising messages of all kinds did not work in ways other than just "real time" ... there would be no such thing as brand/image advertising on TV.
Same applies to the Web as to every other marketing category, distribution channel, and media vehicle. People collect information, and are influenced, over time, even including post-purchase, when many individuals ironically do more research than they did prior to purchase (contributing to the phenomenon called "buyers remorse").
There is no way to assign a general rule of thumb to your business other than to do the research and try to develop metrics over time that give you a good idea of what works and what doesn't. There may be industry resources that provide clues.
Categories vary wildly in this regard. In autos for example, people use varying combinations across multiple channels (and within channel) to gather information before making decisions on what to purchase and where to purchase it from: advertising across TV/radio/print/Web, visits to car lots, word-of-mouth and so on. Just within the Web, information is gathered from auto sales sites, review sites, forums, ad banners, etc.
In some categories in other words, people research online and buy offline; in some it's the other way around; and in some it's a rather complicated matrix of paths from awaresness to intent to purchase.