I've seen that no contractions are used in formal writing. I believe "when're" tries to represent spoken casual English when it would sound like "when-er". Is "when're" ok in an informal text? For example:

"When're they coming back?"

closed as primarily opinion-based by FumbleFingers, Edwin Ashworth, David, Davo, AndyT Aug 21 '17 at 13:39

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    Welcome to our site! I’m afraid whether it’s “proper” to write what you’ve written is going to be very hard for anyone here to answer authoritatively, rather than merely opinionatively. That’s because there aren’t any widely accepted “rules of propriety” when it comes to attempting to convey the phonetic details of actual speech using writing in English via eye-dialect. Please consider editing your post into something more easily answered here. – tchrist Aug 20 '17 at 16:28
  • Yes, when're is an acceptable contraction. As is who're, what're, how're, why're, although who're could be mistaken for whore. – AmE speaker Aug 20 '17 at 18:37
  • ODO's judgment is that it "isn't terribly efficient", "isn't very useful" , isn't much used but is theoretically possible. – Edwin Ashworth Aug 20 '17 at 18:49
  • There's almost no discernible difference in pronunciation between when're and when are, so I wouldn't bother. But yes, you could write that in informal writing. – aparente001 Aug 21 '17 at 5:36