A method for online advertising, is provided, including: receiving a request for a web page from a client device; in response to the request, selecting an advertisement for presentation on the web page, and transmitting the web page to the client device; determining a duration of exposure of the advertisement when the web page is presented on the client device; determining a level of performance of a guaranteed presentation amount associated with the advertisement, the level of performance based on the duration of exposure of the advertisement. In another embodiment, a plurality of candidate advertisements are identified for presentation on a requested web page; for each candidate, an expected cost per time unit (eCPTU) of exposure is determined; one candidate advertisement is selected for presentation on the web page based on the eCPTU's; the web page is transmitted to a client device.