Inventory forecasts for new ad units
New ad units must be on a site at least 28 days for forecasts to be accurate. Please don't depend on any forecasting numbers you see before 28 days. DoubleClick for Publishers uses a sample of the last 28 days of your network's impression activity as a basis to model future traffic patterns.
You can pre-tag your site to build forecasts before launching campaigns. To report on forecasting data as soon as a campaign starts, pre-tag your site at least 28 days before the campaign launches.