Hi Helder, good question. Normally we say 'now' instead of 'here'. That is a special idiom, giving the sense of both time and space. I take 'here' to be describing the current situation. That's my best understanding. If you want to say that you will leave at some time in the next 30 minutes, probably best to say something like "I'll leave within a half an hour."

(This is according to my U.S.-native-speaking intuition). I can't think of any normal example where "here" is used to mean "the present time." No, you can't say "I'll leave from here to half-an-hour." Nor can you say "I'll leave from now to half-an-hour." The most natural phrasing is "I'll leave sometime within the next half-hour." Or "I'll leave sometime between now and 8 p.m." In both cases, you could leave out the word "sometime," and in the first one you could say "in" instead of "within." "From here to eternity" is rather interesting. It isn't a normal phrase. And it isn't really a idiom. It's special. I think the origin is Rudyard Kipling's poem, "Gentlemen-Rankers," with the refrain: "We’re poor little lambs who’ve lost our way, Baa! Baa! Baa! We’re little black sheep who’ve gone astray, Baa—aa—aa! Gentlemen-rankers out on the spree, Damned from here to Eternity, God ha’ mercy on such as we, Baa! Yah! Bah!" There's a novel entitled "From Here to Eternity" (and a movie based on it), and the writer, James Jones, credits the phrase to "Gentleman-Rankers." (It was adapted into a famous Yale College song, "The Whiffenpoof Song," which is where most U.S. speakers have heard it). Now, why Kipling chose those exact words, I don't know--but keep in mind that it's poetry and he may have been thinking about the sounds of the words.

I agree that it would be "from now until then" (not here), but it just does not sound natural in your particular sentence example. "I'll leave within the next half hour." "I'll leave sometime in the next half hour." "I'll leave sometime between now and then." I could say, "I'll leave in thirty to forty minutes" but I cannot seem to make the "from ___ to ___" work naturally. Maybe the word "from" is the awkward one. "From here to eternity" might be a bit more poetic than everyday usage.

You could also say "in" instead of "within." "I'm leaving in thirty minutes," or "I'm leaving in the next thirty minutes." This is if you are definitely sure that you will be leaving in that time frame. If not, you could use other future forms, like "going to," "will," or "will be leaving."

Another expression using 'here' for 'now' is 'from here on', which means 'from this point in time forward' http://idioms.thefreedictionary.com/from+here+on . 'Here' is only used to mean 'now' in set expressions.