Just curious, but why can't you use "ma finche' quando"?

Duolingo tells me correct answer is.... 'sì, ma fino a quando?' It was rejected when I missed out the 'a' Can anyone tell me why?

I was wondering the same and from what I've seen, it seems "Sì, ma finqué quando?" means more or less the same thing. However, I am also wondering if the concept of time is already included in "finqué" and quando is not needed here (?). In other words, is "fino a quando" more or less the same as "finqué quando", or as "finqué"? ..Or are they different? Thanks!

Why is "...entro quando" rejected?

is finche bein used in affermative sentences? and a negative?

I read this whole thread, and still don't get it. I wrote, "Si, pero finchè quando?" I missed it, according to the DL underline hints, because of "a fino". DL said I should have said "Sì, però fino a quando?" I don't understand why "finche"--which means "until"-- is wrong, and "fino a" is needed in this case. I found this, if it helps others, but I'm still not getting it.

https://www.duolingo.com/comment/1884889/Finche-vs-Fino