Say you have a countdown timer, similar to: https://pauljadam.com/demos/ariacountdown.html
The timer indicates when the page will time out and you would have to refresh the page or start again. It would be a 20 minute timer.
After 18 minutes a dialogue could appear informing the user. The user could choose to extend the session without needing to refresh the page, which could result in having to re-input details.
However, if the timer is present on the page at all times and not dismissible, would it be in breach of 'pause, stop, hide' criterion: https://www.w3.org/WAI/WCAG20/Understanding/pause-stop-hide
From the SC: "...unless the auto-updating is part of an activity where it is essential." In other words, if the timer is essential to the page, then you don't need to provide a way to stop it. For something similar to what you describe above, I would say a time out indicator could be considered non-essential, but it will depend on the context.