By default, the timer will display on product pages.
This is the most popular place as utilised by Amazon and many other merchants.
If you want to also output the timer in the cart page, you can do so by inserting the following script into your theme’s cart liquid or equivalent section/snippet:
Doing the above will mean the timer is now also outputted on the cart page. If you wanted to hide it outputting on the product page, you could use product page targeted CSS to do so.
Finally, you could also chose to output it in the header. If you do this, it will show everywhere in your store except the checkout.