Swap requests are canceled automatically if they are not completed within 7 days.
You can check how long is remaining until the swap will be canceled by looking on the list in your Request Box, where it says "~ days until cancellation."
If there is a problem with the request, or for some reason the swap is no longer possible, you can also select a refusal message and choose to cancel the swap.
Swaps that have been canceled automatically, or that have been declined either by you or the other user, can be seen by selecting "Settings" and then "Swap History", and looking under the "Unsuccessful Swaps" tab.
You can also view your Swap History by tapping the History icon in the upper-right corner of the Request Box.