Can a chat trigger fire based on how long a chat has been active?
There are no conditions that reference how long a chat has been active.
However, you can have the trigger run based on when a customer requests chat, and then check if they are still on the site or page after a certain amount of time:
This solution won't always be ideal, as the trigger could theoretically fire in situations where the customer has ended the chat but are still on the site.
See What is the difference between "Still on site" and "Still on page" condition for Chat triggers? for more on those two conditions.