51:5 Let’s say that someone ate half a k’zayis, waited a bit, then ate another half a k’zayis. Do they still combine to form a full k’zayis? That depends. If the time in between the two half measures was within “k’dei achilas pras” – that is, the standard length of time that it takes to eat something, they combine and he recites a concluding bracha. Longer than this, and the items no longer combine. Drinks do not combine even if the delay was less than “k’dei achilas pras.” (The measure of “k’dei achilas pras” is four medium or three large eggs; this might be as long as 9 minutes depending on the authority consulted.)
51:6 A person who slowly sips a hot beverage like coffee or tea, of which one does not drink the minimum volume in one shot, does not say a concluding bracha even though this is the normal way to drink such a thing.