Formal:
* ਕਦੋਂ ਵਾਪਸ ਆਉਣਾ ਹੈ? (kadon vapas auna hai?) - This literally translates to "When is the return?" and is the most formal way to ask.
* ਵਾਪਸ ਆਉਣ ਦਾ ਸਮਾਂ ਕਦੋਂ ਹੈ? (vapas auna da samaan kadon hai?) - This translates to "When is the time to return?" and is also quite formal.
Informal:
* ਕਦੋਂ ਵਾਪਸ ਆਵੇਂਗਾ? (kadon vapas aavega?) - This translates to "When will you come back?" and is more informal.
* ਕਦੋਂ ਆਵੇਂਗਾ? (kadon aavega?) - This is even more informal and simply asks "When will you come?" You can also use this if the context is clear that you are asking about their return.
Using other phrases:
* ਵਾਪਸ ਆਉਂਦੇ ਸਮੇਂ (vapas aunde samay) - This translates to "At the time of returning" and can be used in a sentence like "ਵਾਪਸ ਆਉਂਦੇ ਸਮੇਂ ਮੈਨੂੰ ਦੱਸ ਦਿਓ (vapas aunde samay mainu dass deo)" meaning "Let me know when you are coming back."
* ਮੁੜਨ ਵੇਲੇ (mudan vele) - This means "at the time of returning" and is a more poetic way to say it.
The best choice depends on the context and your relationship with the person you are speaking to.