מאי חנוכה, דתנו רבנן, בכ"ה בכסליו יומי דחנוכה תמניא אינון, דלא למספד בהון ודלא להתענות בהון (מגילת תענית, משנה כ"ג). שנכנסו יוונים להיכל, טימאו כל השמנים שבהיכל וכשגברה מלכות בית חשמונאי ונצחום,בדקו ולא מצאו אלא פך אחד של שמן, שהיה מונח בחותמו של כהן גדול, ולא היה בו אלא להדליק יום אחד.נעשה בו נס, והדליקו ממנו שמונה ימים. לשנה אחרת קבעום ועשאום ימים טובים בהלל והודאה". (גמרא שבת כא
The question: If the oil was sufficient for one day's lighting anyway, then the nes was only for 7 days, not 8. So why establish 8 days of Chanukka?!
This is a useful question for rhetorical purposes, for those wanting to introduce the idea that it wasn't because of any nes but rather because of the late Succot they celebrated according to II Maccabees. But let us leave aside the controversy, whether or not there even is one. Is this really such a great question?
To my mind, only an anal-retentive accountant could ask this question and really be concerned. Let us assume, for the purpose of argument, that there were no other reasons for the 8 day holiday but the nes -- ignore the 8 day Chanukkat haMizbeach described in many sources, including traditional Jewish sources, and ignore the connection to Succot even in traditional Jewish sources. Let us pretend for a moment that it is just the 8 days of burning oil.
Let us say such a nes occurred, and Chazal saw this and decided to establish an 8 day holiday. All of a sudden, some tzurba meiRabbanan gets up and objects as follows: "Sure, it burned for 8 days due to a miracle. But it would have burned for one day anyway. So we should only establish an 7 day holiday!"
My guess is that they would have rolled their eyes, and perhaps kicked him out of the Beit Midrash just as was done to the ever-so-precise Rabbi Yirmiyah. (See Bava Metzia 21a and several other locations in Shas.) They would say, "Due to a miracle, it burned for this eight day period, so we will establish it for the timespan that it burnt!" Such is obvious.
Of course, I doubt that even the Bet Yosef was really troubled by the question. He asked the question for the purpose of offering his answers, in order to clarify the nature of the nes. And many, many others have offered answers as well.
While not troubled by the question, I find two resolutions/clarifications particularly useful:
1) Since oil sufficient for 1 day lasted 8 days, that means that every day it was burning more slowly, taking up less oil, so all days' burnings were miraculous. Such seems rather straightforward from a plain reading of the gemara, such that the question is a non-starter, even if it were serious.
2) According to the Hebrew Scholion of Megillat Taanit, the reason for 8 days for the Chanukkat haBayit, even though two known previous dedications were for 7 days (namely, Moshe for the Mishkan and Shlomo for the Bet HaMikdash) was that that was how long it took them. Thus, the miracle of the oil was the reason for establishing it on subsequent years, but the 8 day period was for various practical reasons. (This may or may not agree with I or II Maccabees, since both describe this as an 8-day Chanukkat haMizbeach, but the purification and repair was quite likely done previous to this.) Thus, the particular length of the miracle, would be irrelevant since the Yom Tov is patterned after the first one, which was 8 days for whatever reason.
That "whatever reason" can be Succot, but that is only mentioned in II Maccabees, not in I Maccabees or Josephus, (though could well be the reason of Bet Shammai's kineged parei haChag) and then one can ask the same question on the Succot reason as skeptics have asked about the nes Chanukka -- how come I Maccabees has no record of this, and how come Josephus does not record this? Also, II Maccabees contains much expansive material, including miracles which the skeptics among us wouldn't accept. Also it was written centuries after the event. True, it was an abridgment of a set of 5 books (and letters) which may (or may not, it is actually up for debate among scholars) have been written at the time of Judah the Maccabee, but for all we know this is a later insertion, dating as late as 80 CE, spicing up the chag and explaining why they had it for 8 days instead of 7 (as we would expect from other dedication ceremonies), giving some personality and motivation to the account. (That they remembered how they had lived like animals a short while ago during Succot, unable to celebrate in the Temple, whereas now they had access to the Temple.)
The question of why an 8 day dedication ceremony rather than a 7 day one strikes me as a much stronger question than the one about the miracle.