By when do you want to have done it?

[GhotiO |fɪʃ|]

Does B's line work here as a question?
First, I'm interested in "by when", it's not a phrase that you can hear in English every day.
And second, I'm interested in the perfect infinitive, I feel like the simple infinitive is simply not "fit" for this job since losing twenty kilos of weight is not an easy task at all and it certainly takes time, while my question is about the moment of the finalization of this journey.

A: I want to lose twenty kilos.
B: By when do you want to have done it?

 

[GhotiO |fɪʃ|]

The double translation (>>) suggested these options (link).
I don't understand why it didn't suggest any option with the perfect infinitive. Maybe, that's just not what you say, idk.

• By what time do you want it done?
• By what time do you want to do it?
• By what time do you want to get it done?
• By what time do you want to have it done? (even this one is not a perfect infinitive )
• By what time do you want to be done?
• By what time do you want to be finished?

 

[SoothingDave]

"By when?" is a sufficient question.

Don't use "time." That makes it sound like you expect an answer like "By 8 o'clock tonight."

 

[jutfrank]

A: I want to lose twenty kilos.
B: By when do you want to have done it?

Yes, grammatically that's good. As you say, the perfect aspect works well to express the sense of completion that you want.

As SoothingDave says, it's easier just to ask By when? Also, it's natural to put the preposition at the end:

When do you want to have done it by?