Various applications allow you to receive email notifications based on user interactions. Below lists applications and how long you have to use the embedded links before they expire.
Email verification on user signup
Email verification is an optional (but recommended) setting available in user -> profiles. The verification link will be active for 72 hours.
Email list sign up
After a user signs up for a mail list, they will receive a list verification (this actually comes from Constant Contact). This verification link will not expire.
Calendar approval
If a site requires approval for user-submitted calendar events, the Calendar admin will receive an email with a link in order to approve or deny the event. This link will remain active for 48 hours.
Comment approval
You can Approve/Deny a comment from the email up to 24 hours after the comment is submitted. After 24 hours clicking a link will result in a "Token has expired" message. Comments can still be moderated in the admin after 24 hours.
Editorial content approval
If a site requires approval for user-submitted editorial assets, the Editorial admin will receive an email notifying them that something has been submitted. There is no current way to approve or deny the content from within the email.
Business claim approval
If a site requires approval for business claims, the designated admin will receive an email notifying them that something has been requested. There is no current way to approve or deny the claim from within the email.
Upsell approval
If a site requires approval for upsells, the Upsell Package admin will receive an email notifying them that something has been requested. There is no current way to approve or deny the claim from within the email.