In the official mobile clients, the self-destruct timer is available for any photos or videos sent in private chats.
To set the timer on Android, tap the stopwatch icon at the bottom of the fullscreen preview.
To do so on iOS, long press Send arrow in the fullscreen preview and select
'Send with Timer'.
The countdown starts the moment your recipient opens the photo or video you sent — as soon as the timer runs out, this photo or video is removed from both devices.
The desktop clients don't support this feature right now; to send and view self-destructing media, please use the official iOS and Android apps.