"什么时候" means verbatim "what time"
But, when i add 都 (adverb meaning "all") and 了(particle indicating either change of state or completion), i can not grasp what meaning is conveyed by this phrase.
Is there a sentence structure "都。。。。。了", that would shed some light on the semantics or some other explanation ?
都。。。了 indicates past/completion of an event with an emphasis on time, and sometimes it's equivalent of don't you see
都（下午2点）了，你还没吃午饭呐？ You didn't have your lunch yet? Don't you see it's 2 PM already?
It is used to make a(n angry) comment on how late something has happened.
你看，都什么时候了，你怎么现在才来？
Look, how late it is, and you have only arrived now?
or simply:
这都什么时候了！
How late!
"都 ... 了" is a common structure.
That means "... have done" or "it's [time] now" and is always used when the speaker is not satisfied or just angry.
(1) It has a implicit meaning of blame for what you are doing.
Example:
天哪，都八点了你怎么还没去上课？
-- Gosh! It's 8 o'clock but you haven't go to school yet?
(2) But sometimes we also use it when the speaker supposes you did things at a time that is different from your habit.
Example:
活都干完了还不走？
-- Your works have been done but you are still staying here?
Notice that you can omit the "都" in such sentence.
都。。。。。了 basically emphasize the current state.
Like 「我都已經開始做飯了，你們怎麼吃零食？」 (I started cooking, how come you are still eating snack?). In this case, cooking is not completed, yet you can still use 都。。。。。了.
In essence, it is same as 「我已經開始做飯，你們怎麼吃零食？」, without 都。。。。。了 (I started cooking how come you are still eating snack?)
Bear in mind that Chinese normally does not address tense, so the text like it does not necessary mean completion.
In the context ,都 is not refer to "all"。
"都...了" represent the level has already too high。
