This is according to the folks at Android Police who managed to obtain some information from a test preview version of Allo. From what they have learnt, Allo will have an Incognito chat mode which is basically a more privacy-focused aspect of the app, and within Incognito mode, chats can have an expiry.
This means that on top of keeping the chats incognito, your messages won’t stick around forever either and users can choose how long they want their chats to last before being deleted. Users will be able to choose between messages lasting 1 minute, 1 hour, 1 day, 1 week, or off. The times can be changed, but the changes will only apply to future messages and older messages will expire based on whatever the previous timer was set.
However based on this test preview, it does not show how long a message has left before it disappears which isn’t very intuitive, but like we said, this is a test preview so maybe the final build will have a better UI. No word on when Google plans on launching Allo, but hopefully it won’t be too long.