WhatsApp rolls out the ability to delete sent messages, but you'll have to be quick about it

The only caveat would be the fact that WhatsApp will only recall messages up to seven minutes after sending them, which is why we said that you'll have to be quick about it. If successful, the messages will be replaced with a “This message was deleted” notice.
Keep in mind that WhatsApp hasn't yet rolled out the new feature to all users, so it seems that this is more of a phased released, which means it will take some time to arrive on all devices.
source: The Next Web, WhatsApp FAQ
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