WhatsApp said you would see an opened receipt in the chat once the recipient has viewed the photo or video, provided they have read receipts turned on.
If you don’t open the photo or video within 14 days of it being sent, the media will expire from the chat.
You must select view once media each time you want to send a view once photo or video.
WhatsApp warned that view once media can be restored from backup if the message is unopened at the time of back up.
If the photo or video has already been opened, the media will not be included in the backup and can’t be restored.
WhatsApp also noted that encrypted media might be stored for a few weeks on WhatsApp’s servers after you’ve sent it.
If a recipient reports view once media, it will be sent to WhatsApp.
It also warned that someone could take a screenshot, screen recording, photo, or video of the media before it disappears.
“You won’t be notified if someone takes a screenshot or screen recording,” said WhatsApp.
“Only send photos or videos with view once media enabled to trusted individuals.”
source : https://bit.ly/2VhAZat