My answer is phrased so that it is readable to those less familiar with some of the concepts surrounding berachot, so forgive me for stating some 'basic stuff' for you as if they are new information. I don't mean to be impertinent.
The gravity of missing a bracha is unfathomable, yet so is the gravity of saying a bracha l'vatala (wastefully).
It is not entirely true that missing a berach is unfathomable, because as "safek brachot l'hakel" (doubtful blessings are treated leniently) (ספק ברכות להקל). Yes, we shouldn't miss berachot, but making a beracha l'vatala is generally considered just a smidgen worse than missing one. See for example Shulchan Aruch OC 210:2 and Shulchan Aruch OC 18:1
I think you are close to answering your own question in regards to your reference to the shiur of time.
An example of some disagreement in the Mishna Berurah 210:19:
(יט) וספק ברכות להקל - היינו באפילו אם הוא בולע לא יברך כיון שאין כונתו לאכילה וכסברא הראשונה. והנה המ"א חולק על השו"ע והרמ"א וס"ל דבבולע לכו"ע חייב לברך אבל כמה אחרונים החליטו עם פסק הרמ"א דהוא ספק ברכה ולא יברך ולכתחלה טוב ליזהר הרוצה לבלוע שיתכוין ליהנות ממנו בתורת אכילה ויברך עליו [ח"א]:
"And doubtful blessings are treated leniently, meaning that even if he swallowed it, he should not recite a blessing since his intention is not to eat and according to the first opinion. And here, the Magen Avraham disagrees with the Shulchan Aruch, and the Rema and holds that if he swallowed it, he is obligated to recite a blessing, but many later authorities have decided with the ruling of the Rema that it is doubtful blessing and he should not recite a blessing. Initially, it is good to be careful, the one who wants to swallow, should intend to enjoy it according to the laws of eating and recite a blessing on it"
So generally, I extrapolate from the many applications of safek beracha lehakel that if you are uncertain whether or not you should recite a blessing, you should not recite it, as long as you stay within the alloted shiur. If you drink a glass of orange juice large enough to warrant boree nefashot, but you are unsure whether you might drink more, then safek berachot lehakel would mean that you rather postpone saying boree nefashot than saying it multiple times within the shiur.