This is a facinating follow up/analysis of an email studied by Analyticsbridge.com.
This provides some basis on why we should not think that the half life of an email much shorter than snail mail (typically there is general understanding that in customer service, for example, an email is supposed to be answered in http://www.blogger.com/img/blank.gif 24 hours).
Perhaps while customers will expect the company to respond Within a day or two, they may take lot more time to respond to an offer! An asymmetric behavioral expectation. So just going by your experience or gut feeling may not be the right thing unless you put yourselves in the shoes of the consumer.