When you start getting into marketing attribution and reporting on how marketing campaigns have influenced pipeline and deals, inevitably you're going to start talking about payback windows. These usually come up because some over-eager campaign strategist or executive starts asking about the ROI of a campaign that just launched. There is no ROI yet, so we start talking about how it's too early to track ROI and that we won't be able to track ROI for a few months. Thus, the beginning of the conversation of a "payback window".