The change will go into effect on March 29, 2017.
If you track and report on view-through conversions for display or video campaigns in AdWords, you may start seeing much lower numbers starting in April. As of March 29, 2017, the AdWords default view-through conversion window will shrink from 30 days to one day, Google announced Monday.
The change will apply to all conversion actions added after March 29. All existing conversion actions that are set to the default conversion window will see the default change to 1 day.
The change means only conversions that occur within one day of an ad impression will be reported in AdWords as view-through conversions (VTCs). View-through conversions are counted when a user converts after a display or video ad impression is viewable but not clicked on. The last viewable display or video ad impression gets credit for the VTC only when a user has not interacted with an advertiser’s other ads.
A reminder that view-through conversions are not included in the Conversions column in AdWords. This change will only affect VTC column reporting.
Shrinking the default attribution window for view-through conversions to just one day will likely make sense for most advertisers. "Source: - http://marketingland.com/google-adwords-view-conversion-window-will-soon-default-1-day-208467"As Ben Wainberg, manager of display marketing at Jet.com, tells Google, “We’ve found ad views are more likely to lead to a purchase within one day, when our message is fresh in customers’ minds.” Advertisers will still be able customize the window setting for each conversion action from Conversions under the Tools tab.