In GA, the Campaign Timeout is configured under Tracking Info -> Session Settings for the property, and it means that when a user returns to the site within the timeout period, their new session will still be attributed to the same campaign as their initial visit.
The default Campaign Timeout value is six months, which means if you get a user to your site from one campaign, all subsequent visits within the next six months for that same user will be attributed to that initial campaign, even if they've been returning to your site from subsequent emails or social posts or other campaigns.
That's my understanding, anyway. We send multiple emails per week, so I've set this setting to 4 days, but I'm looking for other experiences and wisdom if you have any to offer!