First, you can set a Duration for the Retargeting Audience. This box can be found when you create a new Audience:
In this setting, you set the number of days you want the visitor to remain on the Retargeting Audience. In this example we've used 30 days. That means that any visitor who hasn't been to your site in more than 30 days will be removed from the Retargeting Audience. This is a great way to keep your Audiences full of only visitors who have recently navigated to your site. Keep in mind that the more time that passes, the more likely the user is going to clear their cookies, change browsers, etc, and will remove themselves from your Audience. We recommend starting with 30 or 60 days until you get a good feel for your data.
You can also change Targeting Lookback windows within your campaign settings. When creating or editing a campaign, look for this box on the step where you select your Retargeting Audiences:
Here you can set a Targeting Lookback Window for each Audience. This differs from the Duration mentioned above. In this example, we have the windows set to 7 days. That means the campaign will only serve ads to users in the Audience who have been to your site in the last 7 days. If it's been longer than 7 days the user still remains in the Audience, but they won't be eligible to see your ads.
Changing your Targeting Lookback Window in this manner can be a very effective way to boost your CTR. You won't be spending money showing ads to people who haven't been to your site in over a month and maybe don't remember or aren't interested anymore. Your ads will be in front of the newest visitors most likely to click back to your site.
The added benefit to this is that the user will remain in your Audience, so you can still use that data in different ways for your other campaigns!