There is a debate among the Halachic authorities as to the quantity of a beverage that one must drink to be obligated to recite a Beracha Aharona. The accepted view is that one must drink a Rebi’it, which is equivalent to approximately 3.1 ounces, all at once, as opposed to if one sipped the beverage slowly. If one drinks this amount of water all at once, for example, then according to all views, he must recite "Boreh Nefashot," and if one drinks a Rebi’it of wine or grape juice, then according to all views, he must recite "Al Ha’gefen." However, some authorities maintain that one recites a Beracha Aharona even after drinking smaller quantities (a "Ke’zayit"). And although Halacha does not follow this view, and thus one does not recite a Beracha Aharona if he drank less than a Rebi’it, it is preferable to avoid this debate. Therefore, a person who plans on drinking less than a Rebi’it should try to sip the beverage slowly, rather than drink it all at once. This way, he will not have to recite a Beracha Aharona according to either of the two opinions.
After reciting Kiddush or Habdala, one is required to drink a "Rob Rebi’it" (the majority of a Rebi’it), or approximately 1.7 ounces. In the case of Habdala, or Kiddush on Shabbat morning when one will not be eating bread after Kiddush, drinking this amount creates a situation where the requirement to recite a Beracha Aharona is subject to the aforementioned debate among the Halachic authorities. Therefore, it is recommended after Kiddush on Shabbat morning and Habdala to drink an entire Rebi’it. Despite the fact that the obligations of Kiddush and Habdala require drinking only a "Rob Rebi’it," nevertheless, one should drink an entire Rebi’it – 3.1 ounces – in order to avoid the debate regarding Beracha Aharona. This Halacha also applies when one drinks from the cup of wine over which he recited Birkat Ha’mazon. In this situation, too, one should drink an entire Rebi’it, even though strictly speaking it suffices to drink a "Rob Rebiit."
These Halachot are taken from the work Orah Yamim, Hilchot Berachot, p. 62 (listen to audio recording for precise citation).
Summary: One who drinks a beverage recites a Beracha Aharona only if he drinks 3.1 ounces all at once (as opposed to slowly sipping). However, since some authorities require reciting a Beracha even if one drinks smaller quantities, it is preferable when drinking less than 3.1 ounces to sip the drink slowly, so one does not have to recite a Beracha according to either opinion. After reciting Habdala, and after reciting Kiddush on Shabbat morning if one will not be then eating bread, one should drink 3.1 ounces of the wine or grape juice.
